LHCb - Large Hadron Collider beauty experiment

Time 2021-11-20 13:23:04

Web Name: LHCb - Large Hadron Collider beauty experiment

WebSite: http://lhcb-public.web.cern.ch

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Hadron,Large,LHCb,experiment,beauty,Collider,CERN,EuropeanOrganizationforNuclear

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keywords:CERN, European Organization for Nuclear Research, Physics, Particle, Particles, Laboratory, Science, Accelerators, Accelerator, Collider, Colliders, Large Hadron Collider, LHC, LEP, Experiments, ALICE, A large Ion Collider Experiment
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LHCb - Large Hadron Collider beauty experiment  LHCb staff pages  Physics  Detector  Data collection  Collaboration  Installation LHCb is an experiment set up to explore what happened after the Big Bang that allowed matter to survive and build the Universe we inhabit today

Fourteen billion years ago, the Universe began with a bang. Crammed within an infinitely small space, energy coalesced to form equal quantities of matter and antimatter. But as the Universe cooled and expanded, its composition changed. Just one second after the Big Bang, antimatter had all but disappeared, leaving matter to form everything that we see around us — from the stars and galaxies, to the Earth and all life that it supports.

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19 October 2021: New tests of lepton universality show the same pattern as deviations seen in previous results. Test of lepton universality using B⁰K_S⁰l⁺l^- and B⁺K*⁺l⁺l^- decays.

Today at the CERN seminar and at the Rencontres de Blois the LHCb Collaboration presented new tests of lepton universality, one of the basic principles of the Standard Model (SM) of particle physics. This principle states that the SM treats the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses. The new measurements, while of limited statistical sensitivity, exhibit the same coherent pattern of deviations from lepton universality as seen in previous LHCb results.

In March 2021, the collaboration reported an evidence at 3.1 standard deviations () for the breaking of lepton universality in a measurement of the ratio R_K. Deviations at the level of 2.2-2.5 in a measurement of the ratio R_K*⁰ were also reported in April 2017. The ratio R_K describes how often a B⁺ meson decays to a charged kaon and either a positive and a negative charge muon (K⁺μ⁺μ^-) or a positron-electron pair (K⁺e⁺e^-). The ratio R_K*⁰ is similar, only the meson K⁺ is replaced by a K*⁰ meson in the B meson decay. Both ratios exhibit a deficit of μ⁺μ^- pairs with respect to e⁺e^- pairs. Today’s results also report tests of lepton universality, using ratios R_K and R_K*. However, in this measurement, K and K* mesons with different electric charges have been used. Physicists call these newly measured decays the “isospin partners” of the previously measured decays. The new ratios are labelled R_KS⁰ and R_K*⁺.

LHCb has studied a number of other such ratios comparing decays with different leptons (l=e, μ, or τ leptons) in beauty particle decays (see R(D^*), R(J/ψ) and R(pK)). These results revealed hints of deviations from lepton universality, none of which is significant enough to constitute an observation of new physics on its own. However, according to theorists who study possible extensions of the SM, these deviations combined suggest an interesting and coherent pattern, which could be evidence for physics beyond the SM.

The decays used to study lepton universality are extremely rare. They involve the transformation of a beauty quark into a strange quark (b→s), a process that is highly suppressed in the SM and can be affected by the existence of new particles. These not discovered particles could have masses too high to be produced directly at the Large Hadron Collider. The diagram to the left shows SM contributions to the B⁰K_S⁰l⁺l^- decay, involving photons (γ), W, and Z⁰ bosons. The diagram to the right shows a possible new physics contribution to the decay from a hypothetical leptoquark (LQ) that, unlike the SM bosons, could have different interaction strengths with the different types of leptons. The measurements announced today are also expected to be affected by the same potential new physics contributions.

The top left image above shows the K_S^0+- mass distribution and the top right image the K_S⁰e⁺e^- mass used to determine the R_KS⁰ ratio. The bottom left image shows the K_S^{0+ +-} mass distribution and the bottom right the K_S⁰⁺e⁺e^- mass distribution used to determine the R_K*⁺ ratio. Note that a K*⁺ meson decays into a K_S⁰ and a ⁺ meson and note also a clear accumulation of events at the B⁰ or B⁺ mass.

The values of the R_KS⁰ and R_K*⁺ ratios announced today are listed at the top of this article for different regions of the dilepton invariant mass squared, q². They are individually consistent with the SM at the 1.4 and 1.5 level, their combination at the 2 level (for experts: using a fit for a single Wilson coefficient). It is interesting to note that the central values exhibit the same coherent pattern of deviation as the other lepton universality tests performed by LHCb – the muon pairs are observed less frequently than the electron pairs. The image to the left lists the new measurements by LHCb, the most precise to date, of R_KS⁰ and R_K*⁺ ratios, and previous measurments by Belle.

Read more in the LHCb presentations at CERN and at Blois, in the LHCb paper, in the CERN Courier article and in the conversation article.

10 September 2021: The first LHCb measurement of the W boson mass. [ m_W=8035423_stat10_expsys17_theory9_PDF MeV]

Last week the LHCb Collaboration submitted for publication a paper that reports the first LHCb measurement of the W boson mass with an uncertainty of 32 MeV using the 2016 data set. A more precise measurement with a total uncertainty of ≲ 20 MeV looks achievable with already existing LHCb data.

One of the common decays of a W boson is into a muon and a neutrino. The muon transverse momentum distribution, or better 1/p_T, was selected as the best way to measure the W boson mass, m_W, in this analysis. The shape of this distribution is related the value of m_W but it is also strongly influenced by the W boson transverse momentum distribution, p_T^W, the modelling of which is a potential source of a limiting systematic uncertainty. At the LHC W bosons are produced in the collisions of partons (quarks, gluons) carrying a fraction x of proton momentum encoded in the parton distribution functions, PDFs.

The left image above shows the q/p_T distribution, where q is the muon charge. In this way muons from the decay of positive W⁺ and negative W^- bosons are seen in different part of the image. LHCb physicists decided to use measurements of the transverse momentum distribution for Z boson production, p_T^Z, in order to validate (calibrate) the predictions for the p_T^W distribution. The right image above shows the distribution of the ^* variable, defined in the paper, and strongly related to the p_T^Z distribution. The W boson mass is measured in the simultaneous analysis (fits) of the q/p_T distribution of muons from the W boson decays and the ^* distribution from the Z boson decays. Note that the Z boson mass is known with the very high precision of 2.1 MeV. This method has strongly reduced sensitivity to the uncertainties in modelling the W boson transverse momentum distribution. The W mass was measured with the three different parton distribution functions, PDFs. Treating them equally leads to the following arithmetic average: m_W=8035423_stat10_expsys17_theory9_PDF MeV.

The charged W bosons were discovered, together with their neutral partner Z boson, by the UA1 and UA2 experiments at CERN in 1983. Their experimental discovery was crucial in establishing what is now called the Standard Model (SM) of particle physics. In the SM the values of different parameters are related to each other. Precisely measured values of some parameters fix (predict) values of other not well measured or unknown parameters. In this way the value of the Higgs boson mass was predicted before the Higgs boson was discovered by the ATLAS and CMS experiments at CERN in 2012. The value of the W mass was one of the most important parameters used in this prediction. The W boson mass was measured to a precision of 33 MeV at the Large Electron-Positron (LEP) collider at CERN from a combination of results obtained by the ALEPH, DELPHI, L3 and OPAL experiments and to a precision of 16 MeV in an average of measurements by the CDF and D0 experiments at the Fermilab Tevatron collider. The first measurement at the LHC was performed by the ATLAS collaboration and has an uncertainty of 19 MeV. The current SM prediction of the W boson mass is m_W=803547 MeV, based on the combination of a large number of SM measurements except that of m_W, while the average of direct measurements is m_W=8037912 MeV. Since all the SM particles are now discovered, the differences between the SM predictions and direct measurements are used to test the completeness of the SM and could indicate the presence of the physics beyond the SM if significant discrepancies are identified. The m_W comparison is currently primarily limited by the precision of the direct measurements of m_W. The image above, obtained by the Gfitter group, shows the compatibility of the SM predictions in blue with the direct measurements of the W boson and top quark masses in yellow.

The image to the left shows the comparison of the LHCb result, obtained using the 2016 data set, with those of LEP, Tevatron and ATLAS experiments as well as with the SM prediction marked as "Electroweak Fit". A future analysis will use the full available LHCb data set, with which a W mass measurement could be made with an uncertainty of 20 MeV or better. Further reduction could be obtained in collaboration with theoretical physicists by improving the modeling of the W boson transverse momentum distribution. The combination of LHCb results with those from ATLAS and CMS experiments will reduce the sensitivity to the parton distribution functions, PDFs. In fact, in the unique forward geometry of LHCb the W bosons are produced in collisions of high- with low-x partons, while collisions of mid-x partons mainly produce the W bosons observed at ATLAS and CMS.

Read more in the LHCb paper. These results have been already presented in CERN seminar and EPS conference presentations [1] and [2].

29 July 2021: Observation of an exceptionally charming tetraquark.

This week at the European Physical Society conference on high energy physics, EPS-HEP 2021 the LHCb Collaboration presented the first observation of a doubly charmed tetraquark, T_cc⁺, with a new quark content ccud. The newly discovered particle containing two heavy charm quarks is manifestly exotic, i.e. beyond the conventional pattern of hadron formation found in mesons and baryons. The tetraquark particle manifests itself as a narrow peak in the D⁰D⁰π⁺ meson mass spectrum, just below D^*+D⁰ mass threshold, with a statistical significance exceeding 20 standard deviations. The full Run 1 and Run 2 dataset was used to obtain this discovery.

The image above shows the D⁰D⁰π⁺ invariant mass spectrum. A spectacular peak, representing the T_cc⁺ tetraquark, is clearly visible slightly below the kinematical threshold of D^*+ and D⁰ meson production indicated by the dashed green line. The inset shows a zoom in the T_cc⁺ mass region. Note that D^*+ mesons decay promptly into D⁰ and π⁺ meson pair. The two-dimensional distribution for the mass of one D⁰ meson versus the mass of the other one, shown in the image to the left, indicates high purity of the data sample.

In the conventional quark model, strongly interacting particles, known as hadrons, are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In their fundamental 1964 papers [1] and [2], in which they proposed the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration to form hadrons with four (tetraquark) or five (pentaquark) quark constituents. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)^- particle, first observed by the Belle collaboration, is composed of four quarks (ccud). Then, a major turning point in exotic baryon spectroscopy was achieved at the Large Hadron Collider in July 2015 when, from an analysis of Run 1 data, the LHCb collaboration reported significant pentaquark structures in the J/ψp mass distribution in Λ_b⁰→ J/ψpK^- decays. In July 2020 LHCb physicists announced the possible discovery of a four-charm quark tetraquark. Up to this point all known exotic particles contained a charm-anticharm or an beauty-antibeauty quark pair, in August 2020 the first discovery of an open-charm cdus tetraquark and in March 2021 hidden-charm with strangeness tetraquark and this week a doubly charmed tetraquark.

What is the “internal mechanism” of quark interactions inside tetraquarks? Two possibilities are proposed. The quarks could be tightly bound, or they could also be loosely bound in meson-meson molecule, in which color-neutral mesons feel a residual strong force similar to the one that binds nucleons together within nuclei. The proximity of the T_cc⁺ mass to the D^*+D⁰ threshold naively points to the meson-meson molecule interpretation as shown in the left image. However, an interpretation as tightly bound multiquark structure is also possible as shown in the right image. It is also interesting to note an extremely close proximity of the mass of the enigmatic particle χ_c1(3872) to the sum of the masses of the D⁰ and D^*0 mesons, as shown recently by LHCb. A close proximity of the χ_c1(3872) and T_cc⁺ masses could indicate a deep, not yet understood connection between the two phenomena.

All exotic hadrons observed so far decay via the strong interaction. A long-lived exotic particle stable with respect to the strong interaction would be intriguing for the particle physics community. A hadron with two heavy quarks and two light antiquarks, is a prime candidate. However, it was not clear whether such a particle would exist, mainly due to lack of experimental information about the strength of the interaction between two heavy quarks. The situation changed four years ago when LHCb discovered ([1], [2], [3]) an exceptionally charming particle, Ξ_cc⁺⁺, a baryon containing two charm quarks and one up quark. This LHCb observation allowed theoretical physicists to firmly predict the existence of the stable bbud tetraquark. In the case of a ccud tetraquark, theoretical predictions were less clear and there was no consensus whether such a tetraquark, T_cc⁺, would decay weakly or even exist. The long-awaited discovery this week shows that a strongly decaying T_cc⁺ exists and it is narrow enough to be detected experimentally. This observation further supports the existence of a bbud tetraquark that is stable with respect to the strong and electromagnetic interactions. The experimental hunt for this will continue.

The observation of two more hadrons has also been announced this week at the conference by LHCb, Ξ_b(6327)⁰ and Ξ_b(6330)⁰. Thus three more hadrons have been added to the list, and plot, of hadrons discovered at the LHC bringing the total number of discovered hadrons to 62.

Read more in the LHCb conference presentations [1] and [2], in the CERN seminar, in the CERN update in English and French, in the CERN Courier article, in the Symmetry magazine and in the LHCb papers [1] and [2].

27 July 2021: The charm of a proton.

Today at the European Physical Society conference on high energy physics, EPS-HEP 2021 the LHCb Collaboration presented results of a measurement consistent with the presence of valence-like charm content in the proton, referred to as intrinsic charm. In order to obtain this result LHCb physicists measured the fraction of proton-proton collisions with a Z boson and a jet coming from a charm quark (Z+c-jet) /(Z+jet), where a jet is a large cone of particles produced by a high energy quark produced in the event.

In the quark model protons are composed of three quarks, u, u and d, named “valence quarks”, carring a large fraction, x, of the proton momentum and interacting by exchange of gluons. Gluons occasionally can convert into quark-antiquark pairs, dd, uu, ss and cc. These quarks are named “sea” quarks and they carry a small fraction of the proton momentum. It has been debated for decades if the proton also contains a cc component at large x behaving like valence quarks and referred to as intrinsic charm.

Measurements of c-hadron production in deep inelastic scattering and in fixed-target experiments have been interpreted both as evidence for and against of existence of intrinsic charm component of the proton at the percent level. Today LHCb physicists announced an important turning point in this research by measuring (Z+c-jet)/(Z+jet) in the forward region of proton-proton collisions, see the leading Feynmann graphs above.

The image above shows (Z+c-jet)/(Z+jet) (grey bands) for three intervals of forward Z rapidity, y(Z). In simple terms, rapidity is related to the angle of the particle with respect to the beam axis. A higher rapidity means the Z boson is travelling closer to the proton beam. The results are compared to the Standard Model predictions without intrinsic charm (No IC - blue points), allowing to vary intinsic charm contribution, hence permitting, (green squares) and predicting intinsic charm with a mean momentum fraction of 1% (red triangles). The observed values are consistent with both the no intinsic charm and intinsic charm hypotheses for the first two y(Z) intervals; however, this is not the case in the forward-most interval where the ratio of the observed to no-intinsic-charm-expected values is 1.82±0.25. Indeed, when including a contribution of intrinsic charm to the calculations, with an overall momentum fraction of 1% (red triangles) at values of x around 0.3, the theory predictions are consistent with the data. Similarly, the NNPDF 3.0 predictions that include intrinsic charm (green squares) are consistent with the data as well.

The LHCb results are consistent with the prediction of effects expected if the proton contains a valence-like charm content, however, these results will need to be incorporated into the global analyses of parton distribution functions of the proton before firm conclusions can be drawn about intrinsic charm. The weight of protons dominates the weight of our human body. Philosophers might find it interesting that the human body contains a charming component... and even an anti-charming component.

Read more in the LHCb EPS presentation and in the LHCb paper.

11 June 2021: Branching fraction tension. The B_s→φμ⁺μ^- branching fraction measured below the Standard Model prediction.

This week at the Flavor Physics and CP Violation (FPCP) conference in Shanghai, China, the LHCb Collaboration presented a measurement of the B_s→φμ⁺μ^- branching fraction and the angular analysis of its decay products. A previous branching fraction measurement using the Run 1 data set obtained a tension with the Standard Model (SM) prediction at the level of three standard deviations (σ). The analysis reported this week used the full Run 1 and Run 2 dataset, about four times more decays of B_s mesons, and confirmed the tension, which is now at the level of 3.6σ, depending on the details of the theory prediction.

As reported often at this web page, recent studies of b→sll (l=e, μ, or τ leptons) rare beauty particle decays exhibit tensions between experimental results and the SM predictions of branching fractions, angular distributions and lepton universality. None of these results is significant enough to constitute observation of new physics on their own. However, according to theorists who study possible extensions of the SM, these tensions combined suggest an interesting and coherent pattern.

The image above shows the measured values of branching ratio as a function of the squared invariant mass of μ⁺μ^- pair, q², compared with the SM predictions. Calculating these predictions is particularly challenging. Theorist have to account for the hadronic physics involved in turning a B_s meson into a φ meson following the b→s quark transition, so-called form factors, in addition to hard-to-model interference effects from other hadronic processes. The SM predictions are based on form factor calculations using Light Cone Sum Rules (LCSRs) at low q² and Lattice QCD (LQCD) at high q². In the q² region between 1.1 and 6.0 GeV², the measured branching fraction lies 3.6σ below a SM prediction which used both LCSR and LQCD calculations (or 1.8σ with LCSR calculations alone). This deficit in muons follows the same pattern as observed in lepton flavour universality tests, which is very interesting. However, the difficulty of parameterising hadronic effects in the branching fraction prediction calls for caution, as, unlike in lepton flavour universality tests, the discrepancy seen here could be due to inaccuracies in the SM predictions.

Angular analyses of the B_s→φμ⁺μ^- decays provide complementary information to the branching fraction measurement and allows to better probe the structure of potential new physics. Overall, the results are in good agreement with the SM predictions, with the largest deviation in a single observable being measured in F_L, as shown in the image on the left along with the corresponding SM predictions.

To assess the overall agreement of all the angular observables with the SM, a global fit to the observables was performed using the FLAVIO package. Global fits allow to determine constants (referred to as effective couplings) related to the forces experienced by the particles during the decay. Of particular interest is the effective coupling Re(C9) [for experts: the real value of the vector coupling]. The favoured value of Re(C9) lies below the SM prediction and is preferred over the SM by 1.9 σ, which is consistent with the behaviour seen in B⁰→K^*μ⁺μ^- and B⁰→K^*+μ⁺μ^- angular analyses.

Read more in the conference presentation, in the branching fraction publication and in the forthcoming angular analysis paper.

8 June 2021: First observation of the mass difference between neutral charm mesons. m₁-m₂ = 6.4x10^-6 eV = 0.00000000000000000000000000000000000001 grams (1x10^-38g) (m₁-m₂)/(D⁰mass) = 3x10^-15

Today, the LHCb Collaboration submitted a paper for publication that reports the first observation of the mass difference between neutral charm mesons (or rather their mass eigenstates for experts). The result is also presented today at the CERN seminar and was reported last week at the 10^th International Workshop on CHARM Physics. This mass difference determines the frequency of D⁰-D⁰ neutral charm meson oscillations in which the D⁰ particle composed of a charm quark bound with an up antiquark turns into its own antiparticle composed of a charm antiquark and an up quark. The oscillation phenomenon is a fascinating feature of quantum mechanics. The value announced today represents one of the world’s smallest mass difference between two particles of 6.4x10^-6 eV or, when converted to more conventional units, 0.00000000000000000000000000000000000001 grams (1x10^-38g).

The neutral meson particle-antiparticle systems, B_s⁰−B_s⁰, B⁰-B⁰ and D⁰-D⁰ oscillate with very different frequencies. The B_s⁰−B_s⁰ oscillations are the fastest, about 3 million million times per second (3*10¹²). The oscillations B⁰-B⁰ are about 37 times slower while the oscillations D⁰-D⁰ are even slower; the oscillation period is over one hundred times larger than the average lifetime of a D⁰ meson. Therefore only the first part of the oscillation period is observed experimentally. The image to the left shows the B_s⁰−B_s⁰ very well visible oscillation pattern, the right image shows the beginning of the D⁰-D⁰ oscillation period. Both measuremements were performed by LHCb.

The quantum mechanical treatment of neutral charm meson oscillations leads to two neutral mesons, D₁ and D₂, each with their own mass, m₁ and m₂, and typical lifetime represented by their decay width, Γ₁ and Γ₂. The mass difference, m₁-m₂, determines the frequency of oscillations as shown in this cartoon view of the phenomenon. For neutral charm mesons this mass difference is included traditionally in the dimensionless parameter x= (m₁-m₂)/Γ in which Γ is the average width, (Γ₁+Γ₂)/2. Until today the measured value of x was marginally compatible with zero.

The LHCb collaboration has announced two month ago the precise determination of the heavy and light strange beauty meson mass difference determining in this way the B_s⁰−B_s⁰ oscillation frequency. The LHCb collaboration announced the first observation of the D⁰-D⁰ oscillations in November 2012 by excluding no-oscillation (no-mixing) flat distribution by more than 5 standard deviations in the "discovery" plot shown as the right oscillation image above. Today’s measurement of the mass difference determining the frequency of these oscillations is another important milestone in the neutral meson oscillation history.

LHCb physicists used the D⁰ meson decays D⁰→K_s⁰π⁺π^- in the mass difference measurement. The decays were studied with the help of the so-called Dalitz plot, invented in order to study particle decays into three particles. The axes of the plot are the squares of the invariant masses of two pairs of the decay products, m²(K_s⁰π^±) and m²(K_s⁰π^∓). A joint analysis of the Dalitz plot and decay time distributions allowed to determine the value of normalized mass difference, x, together with other oscillation and CP violation parameters. The value of x announced today is different from zero with a precision of more than 7 standard deviations (see above) and constitutes the first observation of a non-zero value of x. This value determines the mass difference m₁-m₂, and in this way, the oscillation frequency. The image shows the decay time integrated Dalitz plot distribution. In the RS (Right Sign) part of the plot we find events in which D⁰ or D⁰ mesons have not oscillated before they decayed and in the WS (Wrong Sign) part those which oscillated. The analysts divided the Dalitz plot into regions and measured the oscillations separately in these regions. A typical distribution of WS/RS decays is show above. Data (black points with error bars) are not consistent with prediction for x=0 (red points) but are with x of about 4x10^-3 (blue histogram). Note also a horizontal band in the RS side part of the image indicating the presence of intermediate K* meson in the D⁰ or D⁰ decays.

More details can be found in the LHCb paper, as well as, in the seminar and workshop presentations. Read more in the CERN update and in the Conversation article.

12 April 2021: Fascinating quantum mechanics. Precise determination of the B_s⁰−B_s⁰ oscillation frequency. "A phenomenon in which quantum mechanics gives a most remarkable prediction" - Richard Feynman

Today, the LHCb Collaboration submitted a paper for publication that reports a precise determination of the B_s⁰−B_s⁰ oscillation frequency. This result is presented also today at the joint annual conference of the UK Institute of Physics (IOP), organized by the University of Edinburgh. The B_s⁰−B_s⁰ oscillation is a spectacular and fascinating feature of quantum mechanics. The strange beauty particle B⁰_s composed of a beauty antiquark (b) bound with a strange quark s turns into its antiparticle partner B_s⁰ composed of a b quark and an s antiquark (s) about 3 million million times per second (3*10¹²) as seen in the image below.

In order to determine if a neutral meson oscillated into its antiparticle the nature (flavour) of the B_s⁰ or B_s⁰ meson needs to be known both at its production and decay. The B⁰_s particles have been identified through their decay into a charmed-strange D_s^- meson and a pion, B⁰_s→ D_s^-π⁺, while B_s⁰ antiparticles through their decay with opposite charges, B_s⁰→ D_s⁺π^-. The identification at the production is more complicated. A range of inputs is collected at the B⁰_s production point and combined in a sophisticated algorithm in order to determine its nature (flavour). Of course, LHCb observes B⁰_s particles and antiparticles only during their short lifetime in which they travel about 1 cm in the LHCb detector. The image above shows the oscillation pattern of events having the same nature at the production and decay in blue, and those, for which the nature has changed in red. In both cases a spectacular oscillation pattern is observed as a fuction of the B_s⁰ measured lifetime.

LHCb physicists measured precisely the oscillation frequency of these oscillation to be Δm_s=17.7683±0.0051±0.0032ps^-1. A combination of this result with previous LHCb measurements gives the value of Δm_s=17.7656±0.0057ps^-1. The image shows a summary of LHCb measurements. The excellent precision of the LHCb detector, particularly its Vertex Locator (VELO), allowed LHCb to obtain these results.

The neutral meson particle-antiparticle (matter-antimatter) oscillation was predicted by Gell-Mann and Pais in 1950es. They noticed that both the neutral strange meson K⁰, and its antiparticle K⁰, decay into a pair of pions, π⁺π^-. Therefore a K⁰ meson can turn into K⁰ meson and vice versa following the reaction K⁰↔ π⁺π^-↔K⁰. Thus the two particles, K⁰ and K⁰, should be considered in quantum mechanics as a single two-state system. The quantum mechanical treatment leads to the particle-antiparticle oscillation pattern. For experts: the quantum mechanical superposition of the neutral strange meson K⁰, and it antiparticle K⁰, leads to two neutral strange mesons each with their own mass and typical lifetime. A consequence of this coupled system is that the K⁰ meson can turn into the anti-K⁰ meson, K⁰, and vice versa. Richard Feynman explained the Gell-Man and Pais prediction, not confirmed experimentally at that time, on 9 pages in his memorable Lectures on Physics (Vol. III, 11-5) calling this phenomenon “a most remarkable prediction of quantum mechanics”. Gell-Mann and Pais could not predict the parameters of these oscillations since they did not know “internal mechanics” of K mesons. In the quantum mechanical two-state system, the B_s⁰−B_s⁰ system is described also as a system of two mass states, heavy and light. The oscillation frequency corresponds to the minute mass difference Δm_s between these two mass states, amounting to only 0.011 eV (or 2.1 x 10^-35 g when converted to more conventional units).

Now we know not only the strange neutral mesons, but also charm, beauty and strange beauty neutral mesons. We know also internal mechanics of these mesons: the quark model and the Standard Model (SM) of particle physics. We can therefore calculate the frequency of these oscillations, as shown in the Feynman graph to the left. The B_s⁰−B_s⁰ oscillations have the highest frequency of all neutral mesons and the oscillations are the most spectacular. The oscillations B⁰-B⁰ are about 37 times slower; the oscillations D⁰-D⁰ are even slower and therefore only the first part of the oscillation period is observed experimentally, see 7 November 2012 news.

Even more; the oscillation frequency is measured and calculated so precisely that it provides one of the strongest constraints to models beyond the SM, that try to explain the observed flavour anomalies. For example, models with a new fundamental force Z' could easily alter the B_s⁰ oscillation frequency.

The image of the B_s⁰−B_s⁰ oscillations is a beautiful visual example of the quantum-mechanical nature of our universe.

Read more in the LHCb paper and conference presentation. Preliminary results have been already presented at Les Rencontres de Physique de la Valée d’Aoste.

23 March 2021: Strengthened hints for a violation of lepton universality in B decays. Update of R_K measurement

Today at the Rencontres de Moriond EW conference and at a seminar at CERN, the LHCb Collaboration presented an updated measurement of the ratio R_K, an important test of a principle of the Standard Model of particle physics known as "lepton universality". This principle states that the Standard Model treats the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses. An LHCb paper was submitted for publication today. The results indicate evidence for the breaking of lepton universality in beauty-quark decays, with a statistical significance of 3.1 standard deviations. “Evidence” is the term often used in the community for a result that surpasses 3 standard deviations but falls short of the 5 standard deviation level at which an “observation” is commonly claimed.

The ratio R_K describes how often a B⁺ meson decays to a charged kaon and either a positive and a negatively charge muon (K⁺μ⁺μ^-) or a positron-electron pair (K⁺e⁺e^-). These decays are extremely rare, occurring at a rate of only one in two million B⁺ meson decays. The decays involve the transformation of a beauty quark into a strange quark (b→s), a process that is highly suppressed in the Standard Model and can be affected by the existence of new particles, which could have masses too high to be produced directly at the Large Hadron Collider. The left graph shows Standard Model contributions involving γ, W⁺, and Z⁰ bosons. The right one shows a possible new physics contribution to the decay with a hypothetical leptoquark (LQ) which, unlike the Standard Model bosons, could have different interaction strengths with the different types of leptons.

LHCb has studied a number of other such ratios comparing decays with different leptons in beauty particle decays (see R_K, R_K^*0, R(D^*), R(J/ψ) and RpK). These results revealed hints of deviations from lepton universality, none of which was statistically significant enough to constitute evidence of new physics on their own. However, according to theorists who study possible extensions of the Standard Model, these deviations combined suggest an interesting and coherent pattern. The previous measurement of R_K used the LHCb Run 1 and the first part of Run 2 data set. The experimental method applied in today’s analysis is essentially identical but the analysis profits from additional data collected in 2017 and 2018. Measurements like R_K apply a so-called blind analysis, in which the physicists analysing the data do not know the result until the analysis method is finalised and frozen, following an extended review within the collaboration.

To minimise the influence of detector and other experimental effects, LHCb physicists used a "double ratio" method, by measuring R_K divided by another ratio, r_J/ψ. The r_J/ψ ratio is defined as the ratio of probabilities of a B⁺ decay to J/ψK⁺ with J/ψ →μ⁺μ^- vs J/ψK⁺ with J/ψ →e⁺e^-. The true value of the r_J/ψ ratio is predicted to be very close to 1, but it is affected by very similar detector effects as R_K. This gives an extra layer of protection: the scientists study and correct for all known experimental effects, but if any unknown effects slip through they will cancel in the double ratio between R_K and r_J/ψ. Moreover, J/ψ meson decays into μ⁺μ^- and e⁺e^- pairs are known to respect lepton universality at the 0.4% level. This means that the measurement of the single ratio, r_J/ψ, also constitutes an excellent cross-check, since it does not benefit from the double ratio's cancellation of systematic effects and so is a sensitive and stringent test of the methods used to determine the efficiencies. The value of r_J/ψ is found to be 0.981±0.020, consistent with 1. The four figures above show the measured invariant mass distributions of B⁺ candidates for the four decay modes used in the double-ratio measurement, each with a clear accumulation of events around the B⁺ meson mass.

The analysis is performed in the range 1.1q²6.0 GeV², where q² is the invariant mass of the μ⁺μ^- or e⁺e^- pair. The value of R_K is measured to be 0.846^+0.044_-0.041, and is shown in the figure to the left as a black point with error bars. This is the most precise measurement to date and is 3.1σ (3.1 standard deviations) away from the SM prediction, providing evidence for the violation of lepton universality. The previous measurements are shown on the plot as grey points with error bars. The newly measured value of R_K supersedes previous LHCb results. Measurements by the BaBar and Belle collaborations are also shown.

This update is consistent with previous measurements, and with the improved precision the significance has increased past the three sigma level. This result will stimulate further intensive experimental and theoretical efforts. In the near future, LHCb will report on results from the decays of other beauty particles. These will be additional pieces in the puzzle of possible new physics.

Learn more in the LHCb Moriond presentation, in the LHCb CERN seminar, in the LHCb paper, in the CERN media update in English and in French, in the CERN Courier article and in the Conversation article; see also additional explanations by LHCb physicicts [1] and [2] and also follow a discussion.

23 March 2021: Improved measurement of B_s⁰→ μ⁺μ^- decays. [ Branching fraction B_s⁰→μ⁺μ^- = (3.09^+0.46_-0.43^+0.15_-0.11)x10^-9 ; B⁰→μ⁺μ^- 2.6x10^-10 ]

Today, at the Rencontres de Moriond EW and at the CERN seminar, the LHCb collaboration presented an improved measurement of the very rare B_s⁰→ μ⁺μ^- decay. The measured branching fraction (3.09^+0.46_-0.43^+0.15_-0.11)x10^-9 is in agreement with the Standard Model (SM) prediction of (3.66±0.14)x10^-9. It is the most precise single experiment measurement to date.

The full LHCb Run 1 and Run 2 data set was used to obtain this result. A special event selection (BDT for experts) was used to classify data into intervals with different ratios of B_s⁰→μ⁺μ^- decays and background contributions. The μ⁺μ^- invariant mass spectrum for the intervals with the smallest background contribution (BDT0.5) is shown in the image above. The contribution of the B_s⁰→ μ⁺μ^- decay is clearly visible with a statistical significance of 10 standard deviations (σ) at the B_s⁰ mass and is indicated as the red peaking function. The green peaking function at lower mass shows the contribution from the even rarer B⁰→ μ⁺μ^- decay, whose SM rate is about 30 times smaller than that of the B_s⁰→ μ⁺μ^- decay, however, the production of B⁰ is 4 times more abundant than that of B_s⁰. The size of this contribution (1.7σ) is not found to be significant, and so an upper limit is set for the decay at a value of 2.6x10^-10. The other functions show the estimated contributions from background processes, but for the purple function, which represents the B_s⁰→μ⁺μ^- events which lost some energy due to a photon radiated from the B_s⁰ valence quarks. These events, which are experimentally searched for the first time, are found to be compatible with zero within the measurement error, and this allowed to set an upper limit on the rate of B_s⁰→μ⁺μ^-γ at a value of 2.0x10^-9 (95% CL for experts) for μ⁺μ^- mass above 4.9 GeV/c². The left image shows a 2 dimensional representation of the branching fraction measurements for B_s⁰→ μ⁺μ^- and B⁰→ μ⁺μ^-. The SM Model value is shown as the red cross. The central value from today’s measurement is indicated with the blue dot. The blue contours show limits of regions with different statistical proximity (68%, 95% CL, etc., for experts), while the yellow contours indicate the previous, less constraining, measurement. These results are in perfect agreement with the SM prediction.

The probability, or branching fraction, of the B_s⁰ meson to decay into two oppositely charged muons is very small in the SM and is precisely calculated. On the other hand, a large class of theories that extend the SM, such as, for example, supersymmetry, allows significant modifications to this branching fraction and therefore an observation of any significant deviation from the SM prediction would reveal the influence of new physics. The decay of a B_s⁰ meson into a muon pair has therefore long been regarded as one of the most promising places to search for these new effects. This decay has been searched for more than 30 years by different experiments at different accelerators as seen in the image to the left. The LHCb collaboration obtained the first evidence, with a significance of 3.5σ, in November 2012 and, together with the CMS collaboration, the first observation, with a significance of 6.2σ, in May 2015. Moreover, in February 2017, LHCb announced the first single experiment observation, with a statistical significance of 7.8σ. Previous results already severely constrained the type of SM-extension models that are still allowed. The results announced today, with a statistical significance of 10σ and agreement with the SM, isolate even more precisely the parameter region in which these new models can exist, and therefore focuses future experimental searches and theoretical attention. All candidate models of physics beyond the SM will have to demonstrate their compatibility with this important result including models which propose explanation of hints for lepton universality anomalies, like R_K, for which an update was also announced today.

Some new-physics models also allow the possibility of a different B_s⁰ “effective” lifetime from what is predicted in the SM. LHCb also reported today an improved measurement of this quantity, and found it to be 2.07±0.29±0.03 ps in agreement with the SM prediction. The figure to the left shows the distribution of the B_s⁰→μ⁺μ^- decay time, along with the fit function used to measure the lifetime. This function is an exponential decay law, modified to account for the non-uniform detector efficiency.

Learn more in the LHCb Moriond presentation, in the LHCb CERN seminar and in the LHCb papers [1] and [2].

3 March 2021: Observation of two ccus tetraquarks and two ccss tetraquarks. Welcome to tetraquark discovery territory

Today, the LHCb Collaboration submitted a paper for publication that reports the first observation of two tetraquarks with a new quark content ccus, decaying to the J/ψ and K⁺ mesons: a narrow one, Z_cs(4000)⁺, and a broader one Z_cs(4220)⁺. Two other new tetraquarks, X (4685) and X(4630), with a quark content ccss, are also observed. (Here we refer to these exotic hadrons generically as tetraquarks, whereas some experts typically refer to tetraquarks as specific strongly-bound 4-quark states; the exact nature of exotic hadrons is still being debated.)

The four tetraquarks were observed in the decay B⁺→J/ψφK⁺ using the full LHCb Run 1 and Run 2 dataset. A practically pure sample of about 24000 candidates was studied as can be seen in the plot to the left, which shows a large peak in the J/ψφK⁺ invariant-mass spectrum around the B⁺ mass. The decays were studied with the help of a so-called Dalitz plot, invented in order to study 3-body decays. The axes of the plot are the squares of the invariant masses of two pairs of the decay products. The presence of intermediate particles -- formed for a tiny fraction of a second during the B⁺ particle decay -- is revealed through vertical or horizontal bands in what would otherwise be a uniformly-populated plot. The orientation of the band indicates the pair of particles to which the intermediate "resonance" decays. Horizontal bands signal presence of ccus tetraquarks, while vertical bands correspond to the ccss tetraquarks. If the Dalitz plot is interpreted as a tourist map of the tetraquark discovery territory then the North-South mountain ridges represent ccss tetraquarks and West-East ones the ccus tetraquarks.

The study of this decay allows scientists to make exceptional discoveries. Already in June 2016 LHCb has announced observation of four tetraquarks X(4140), X(4274), X(4500) and X(4700) decaying into a J/ψ and a φ meson in the analysis of LHC Run 1 data. The four most apparent vertical bands in the Dalitz plot above further confirm the existence of these ccss tetraquarks in the results presented today.

The possibility of the Z_cs⁺→J/ψK⁺ contributions in the Run 1 data were also tested. Evidence around 3σ was found, but deemed insufficient to claim for the discovery of an exotic particle with a new quark content. With additional Run 2 data a distinct horizontal band at J/ψK⁺ invariant mass squared near 16 GeV² is clearly visible representing the discovery of the Z_cs(4000)⁺ tetraquark in the J/ψK⁺ decay with a mass of 4003±6⁺⁴_-14 MeV and a width of 131±15±26 MeV. The image shows the so called Argand diagram proving to experts that the Z_cs(4000)⁺ structure seen in the data (black dots with error bars) really represent resonant particle production and decay, since it follows approximately a circular path (red circle). The Argand plot study is reminiscent of the previous work by LHCb on non-strange tetarquark candidate, Z_c(4430)^-.

Recently the BESIII experiment reported observation of the structure at the kinematical threshold in the D_s^-D^*0 + D_s^*-D⁰ mass distribution. When interpreted as a tetraquark, called Z_cs(3985)^-, its mass is consistent with the Z_cs(4000)⁺ tetraquark reported by LHCb, but its width is ten times smaller. Therefore it is possible that the Z_cs(4000)⁺ and Z_cs(3985)^- are different particles. Hence we can ask: are there one or two tetraquarks near the D_s^-D^*0 + D_s^*-D⁰ threshold mass? This question will clearly trigger further intensive experimental and theoretical effort. Various interpretations of the tetraquark binding mechanism have been proposed, including tightly bound tetraquark states and loosely bound meson-meson molecules as discussed often on this web page. An interesting possibility would be that both types are produced, one in e⁺e^- collisions at BESIII and the other one in B meson decays at LHCb, with a similar mass and different widths (indicating different lifetimes). In fact there is a previous evidence from the studies of J/ψπ⁺ states, that the narrow ccud states, like Z_c(3900)⁺ are produced in e⁺e^- collisions, and broader states like Z_c(4430)^- are produced in B meson decays, but not vice versa.

In this new LHCb study, a sophisticated "amplitude analysis" approach was used, comparing the data to expectations from various decay hypotheses. This has allowed the discovery of additional exotic states, that are less visible to the naked eye in the plots, and to determine their quantum numbers. In this way a broader Z_cs(4220)⁺ ccus tetraquark was also discovered. Two other new tetraquarks, X (4685) and X(4630), with a quark content ccss are also observed decaying to J/ψφ. The contribution of all the newly discovered exotic particles can be found in the Dalitz plot projections on the J/ψK⁺ and J/ψφ mass axis below. Note the linear mass scales in the projection plots and the mass square scales in the Dalitz plot.

The tetraquarks that were first discovered had masses close to the masses of charmonium states, hence the question of their classification as tetraquarks or standard charmonium cc neutral particles was open. The Z_cs⁺ particles reported today are charged and therefore are unambiguous exotic candidates since they cannot be included in the charmonium family.

In the conventional quark model, strongly interacting particles, known as hadrons, are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In their fundamental 1964 papers [1] and [2], in which they proposed the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration to form hadrons with four (tetraquark) or five (pentaquark) quark constituents. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)^- particle, first observed by the Belle collaboration, is composed of four quarks (ccdu). Then, a major turning point in exotic baryon spectroscopy was achieved at the Large Hadron Collider in July 2015 when, from an analysis of Run 1 data, the LHCb collaboration reported significant pentaquark structures in the J/ψp mass distribution in Λ_b⁰→ J/ψpK^- decays. In July 2020 LHCb physicists announced the possible discovery of a four-charm quark tetraquark and in August 2020 the first discovery of an open-charm cdus tetraquark. The image shows an artist's view of the Z_cs⁺ tetraquark.

More details, including numerical values of new particle properties, can be found in the LHCb publication.

Today’s discovery added four more names to the list of hadrons produced at the LHC. In 11 years of LHC operation 59 hadrons have been discovered by the ATLAS, CMS and LHCb collaborations; on average nearly one every two months. Clearly, the LHC is a Large Hadron Discovery Factory. Read more in the accompanying news and in the CERN Courier article and also in the Conversation article.

3 March 2021: LHC as a Large Hadron Discovery Factory.

The LHC, Large Hadron Collider, is a particle collider in which accelerated protons collide inside four large experiments. Hadrons (Greek: ἁδρός, hadrós, meaning "stout, thick") are a class of particles which the protons belong to, explaining the origin of the word “hadron” in the name of the LHC. In the quark model hadrons are composed of two or more quarks bound by the strong force. They also interact strongly between themselves like in the nucleus of an atom. Hadrons composed of a quark-antiquark pair are called mesons, of three quarks are called baryons, and those composed of larger number of quarks, exotic hadrons.

A very large number of hadrons are produced at the LHC. The excellent LHC detectors are able to reconstruct them efficiently. All the new hadrons that have been discovered at the LHC are shown in the plot above. The colour of the symbol accompanying the hadron name indicates the particle quark content. The hadrons are classified as a function of their mass and year of discovery. In 11 years of LHC operation 59 hadrons have been discovered by the ATLAS, CMS and LHCb collaborations; on average nearly one every two months. Clearly, the LHC is a Large Hadron Discovery Factory. Details of many of the discoveries can be found in the specific news stories on this web page. The list of new particles discovered at the LHC is continuously maintained.

Hadrons discovered in the 1950 and 1960ies, the pioneering years in particle physics history, were called elementary particles till their structure was finally understood in the framework of quark model. The quark combinations can be in their lowest-energy quantum mechanical state: the ground state. Like electrons in atoms, the quarks can be rearranged into excited states with different values of angular momentum and quark spin orientation. Following the particle physics tradition all these quantum states are called hadrons and, more generally, particles.

Four more hadrons were added to the plot and the list today including ccus tetraquarks with hidden charm and open strangeness. The details of this discovery can be found in the accompanying news.

The first hadrons were discovered in cosmic rays and at low energy accelerators. Physicists were analysing practically by hand images of particle tracks recorded in bubble chambers and nuclear emulsions. Today modern technology allows physicist to discover a large number of hadrons with higher masses and sometimes astonishing properties by studying particle collisions at the very high LHC energy, using state of art detectors and fast computers. The discovery adventure continues. Follow it at this web page.

Read more in the CERN update in English and French, in the CERN Courier article and also in the Conversation article.

10 December 2020: More on exclusive vs inclusive puzzle. A measurement of |V_ub|/|V_cb|.

The LHCb collaboration submitted today for publication a paper which reports the determination of the ratio of parameters |V_ub|/|V_cb| involving transitions of a b quark to a u and a c quark. This measurement was made possible with the first observation of the decay B_s⁰→K^-μ⁺ν_μ. Previous |V_ub| measurements performed by the BaBar and Belle experiments at B-factories found two sets of inconsistent results, depending on which method was used to determine the parameter.

The parameter |V_ub| is an element of the 3x3 Cabibbo-Kobayashi-Maskawa (CKM) matrix, see image above. In the Standard Model (SM) the CKM matrix describes the decay of one quark to another by the emission of a W boson. While the SM does not predict the values of the parameters of the CKM matrix, the measurements of these parameters in different processes should be consistent with each other. If they are not, it may be a sign of physics beyond the SM. Within the SM the CKM matrix is unitary which leads to the so-called “unitarity triangle” shown here together with different measurements compiled by the CKMfitter collaboration. The parameter |V_ub| determines a length of the triangle side opposite to the angle β, see a green circle marked “|V_ub|”. The BaBar and Belle collaborations determined the |V_ub| parameter by measuring the decay rates in an “exclusive” way meaning that a specific (exclusive) decay was used and in an “inclusive” way by identifying the electron or muon emitted in the b to u quark transition. The results from exclusive and inclusive measurements showed a discrepancy of about 3σ. Today’s LHCb results bring additional contributions towards the understanding of this long-standing |V_ub| puzzle.

Hadrons containing a b quark can decay via a virtual W boson to semileptonic final states through the transitions b→c and b→u accompanied by a W boson which in turn decays into a charged lepton and neutrino pair. These transitions involve the CKM matrix elements V_cb and V_ub, respectively. LHCb physicists measured the decay rates (branching fractions) of the B_s⁰→K^-μ⁺ν_μ decay, depending on the |V_ub| parameter and the B_s⁰→D_s^-μ⁺ν_μ decay, depending on the |V_cb| parameter. The ratio of decay rates determines the |V_ub|/|V_cb| ratio providing that the, so-called, form factors are known. The form factors describe how a B_s⁰ meson turns into either a K^- or a D_s^- meson and are calculated by theorists. Their value depends on the q², the momentum transfer, or invariant mass squared, of the muon and the neutrino. The LHCb results are presented in two regions of momentum transfer, below and above 7 GeV²/c⁴.

Reconstruction of the two decays is challenging since the neutrinos are not observed in the detector. LHCb physicists therefore reconstructed the B_s⁰ mass in approximation using the concept of the so-called corrected mass m_corr. The images above show the corrected mass distribution for the (left to right) B_s⁰→K^-μ⁺ν_μ decays with q² below and above 7 GeV²/c⁴ as well as for the B_s⁰→D_s^-μ⁺ν_μ decay. The distribution of events used to measure the |V_ub|/|V_cb| ratio are shown as green histograms.

The images above relate the new results to previous measurements. In the left image the |V_ub|/|V_cb| results in the different q² regions are compared to the LHCb Λ_b⁰→pμ^-ν_μ decay measurements, as well as to the Particle Data Group (PDG) world average of exclusive measurements, in which the LHCb measurements are not included. The right image shows the comparison in the |V_ub| vs |V_cb| plane in which results of other experiments are also shown. Today’s high q² result is compatible with the LHCb Λ_b⁰ decay measurement and the world average of exclusive results. The discrepancy between the LHCb values of |V_ub|/|V_cb| ratio for the low and high q² momentum transfer is due to the difference in the theoretical calculations of form factors.

The measurements of decays involving a neutrino are very challenging at a proton collider and it is quite a surprise that these measurement could have been peformed by LHCb. The result presented today as well as the previous measurement with Λ_b⁰ baryons add additional information to the puzzle. However, the picture is not yet complete. Additional joint experimental and theoretical efforts are needed to resolve the puzzle and to explain the remaining difference between exclusive and inclusive |V_ub| and |V_cb| measurements.

Read more in the LHCb paper.

30 October 2020: New data deepen P₅’ mystery. An angular analysis of the B⁺→K^*+μ⁺μ^- decay.

Today, at the yearly workshop “Implications of LHCb measurements and future prospects”, the LHCb collaboration presented an angular analysis of the B⁺→K^*+μ⁺μ^- decay, with subsequent decays K^*+→K_s⁰π⁺ and K_s⁰→π⁺π^-, using the full LHC Run 1 and Run 2 data set. A tension with the Standard Model (SM) is observed. This decay is mediated by the same underlying physics and is sensitive to the same observables as the B⁰→K^*0μ⁺μ^- decay where indications of a tension with the SM emerged.

The analysis of the B⁰→K^*0μ⁺μ^- and B⁺→K^*+μ⁺μ^- decays is a very promising way to search for effects of as-yet-undiscovered particles (see the CERN Courier article for an introduction). At the quark level this decay involves the transition from a b quark to an s quark, accompanied by the production of a muon pair (μ⁺μ^-). The decays are sensitive to virtual contributions from new particles, which could have masses that are inaccessible to direct searches at the LHC. The analysis of these decays is complicated; the best sensitivity to new particles comes from the study of the angular distribution of the kaon, pion and the muons. Physicists from the LHCb experiment have studied different angular observables as functions of the invariant mass squared, q², of the muon pair. A set of observables is analysed that were constructed to be less dependent on a good understanding of the hadronic physics involved in turning a B meson into a K* meson (so-called form factors). It is one of these observables, "P₅’", that showed a local deviation with respect to the SM predictions in one q² bin in the 2013 analysis of B⁰→K^*0μ⁺μ^- decays. In the most recent analysis the comparison with the Standard Model prediction has not been concentrated on one or two bins of the P₅’ distribution. Instead, a global fit was performed to several angular observables using the FLAVIO package. These global fits are interesting because they allow us to determine so-called effective couplings, which relate to the forces experienced by the particles during the decay. Of particular interest is the effective coupling Re(C₉) [for experts: the real value of the vector coupling]. In the B⁰→K^*0μ⁺μ^- analysis, it was found that C₉ deviated from its SM prediction at the level of 3.3σ.

The angular analysis of the B⁺→K^*+μ⁺μ^- decay was performed following an approach similar to that of the B⁰ analysis. The left image above shows the K_s⁰π⁺μ⁺μ^- invariant mass spectrum with a clear enhancement at the B⁺ mass. The right image above shows the P₅’ angular observable as a function of the invariant mass squared, q², of the muon pair. While most of the measured observables agree with SM predictions, local deviations are observed in P₅’ and other angular observables. The discrepancies are in line with those seen in the B⁰ decays, albeit with larger uncertainties. A better description of the data is obtained by a minimal extension of the SM with one free parameter, the Re(C₉). When this model is fit to the measured observables using the FLAVIO phenomenology software, this hypothesis is favoured by 3.1σ with respect to the SM. This is similar to the tension seen in the B⁰→K^*0μ⁺μ^- decay, where the same model was favoured by 3.3σ.

LHCb physicists are systematically studying a principle of the SM known as "lepton universality", which states that the laws of physics treat the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses. Different ratios are investigated that compare beauty particle decays to different leptons (see R_K, R_K^*0, R(D^*), R(J/ψ) and recently R_pK). These measurements revealed hints of deviations from lepton universality, none of which is significant enough to constitute evidence of new physics independently. However, according to theorists who study possible extensions of the SM, taken together these deviations suggest an interesting and coherent pattern. Is there anything common between lepton universality anomalies and angular distribution anomalies reported today? Well, maybe! According to theorists both types of anomalies can be coherently described by the same new physics models.

Other angular distributions and more details can be found in the workshop presentation and also in the forthcoming paper.

28 October 2020: Kπ-puzzle confirmed and strengthened. An unexpected Standard Model twist or manifestation of new physics?

Today, at the yearly workshop “Implications of LHCb measurements and future prospects”, LHCb collaboration announced a result of a measurement of the CP violation in the B⁺→K⁺π⁰ decay using data collected during LHC run 2. The reported asymmetry is the most precise measurement of the CP asymmetry in this decay and exceeds the precision on the current world average. This asymmetry is a key input to studies of a long-standing anomaly in B meson decays known as the Kπ-puzzle.

Studies of the decay B⁰→K⁺π^- by the BaBar and Belle collaborations led to the first observation of CP violation in decay in the B system. Further studies by the BaBar, Belle, Tevatron and LHCb experiments revealed a significant discrepancy between the measured CP asymmetry of the decays B⁰→K⁺π^- and B⁺→K⁺π⁰. However, similar asymmetries are expected based on, so called, isospin arguments. This anomaly is known as the Kπ-puzzle. Today’s LHCb measurement confirms and significantly enhances this anomaly.

The reconstruction of the B⁺→K⁺π⁰ decay is particularly challenging. The LHCb tracking system reconstructs the trajectories of charged particles. Typically beauty particles are produced in a proton-proton interaction accompanied by many other charged particles and are identified by their decay at a distance of about 1 cm into other charged particles, as seen in the B⁰_s→μμ decay. The π⁰ from the B⁺→K⁺π⁰ decay is invisible in the tracking detectors and is reconstructed by the calorimetric system. With only a single charged track being available, no B-meson decay vertex can be reconstructed. Instead, the identification of the B⁰→K⁺π⁰ decay relies on identifying a charged kaon that is inconsistent with originating from the proton-proton collision point but consistent with the reconstructed B trajectory.

The images above show the reconstructed invariant mass distribution of K⁺π⁰ and K^-π⁰ mass distributions. Clear enhancements at the B⁺ (left) and B^- (right) masses are visible. The CP asymmetry between B^- and B⁺ decay rates is found to be A_CP( B⁺→K⁺π⁰)= 0.025±0.015±0.006±0.003. This result is consistent with the world average, and consistent with zero at approximately 1.5σ. The new world average of A_CP( B⁺→K⁺π⁰), including LHCb result, is found to be 0.031±0.013. The corresponding new CP asymmetry difference, a numerical value representing the size of the Kπ puzzle, Δ A_CP(Kπ)= A_CP( B⁺→K⁺π⁰)−A_CP( B⁰→K⁺π^-), is now 0.115±0.014. The significance of the anomalous difference A_CP(K⁺π⁰)−A_CP(K⁺π^-) is strengthened, from 5.5σ to more than 8σ, with respect to simple expectations from isospin symmetry.

An attentive reader of the LHCb news can discover the Kπ puzzle by eye. The two plots above, showing the invariant mass distribution of the K⁺π⁰ and K^-π⁰ mesons, look very similar; no surprise, the measured CP asymmetry is consistent with zero. On the other hand, the two plots at the left, showing the invariant mass distributions of the K⁺π^- and K^-π⁺ mesons, clearly manifest a large CP violation represented by a different height of the B⁰ and B⁰ mass peaks, as indicated by a corresponding horizontal line. These plots come from and are described in more details in the following news at this web page.

Today’s result will increase interest of the theoretical physics community in the study of Kπ puzzle. Attempts to describe this puzzle fully in the framework of the Standard Model were not successful (yet?) and, therefore, different models of the physics beyond the Standard Model are proposed.

Learn more in the LHCb workshop presentation, in the CERN seminar and in the LHCb paper.

6 October 2020: The first observation of time-dependent CP violation in B_s⁰ decays.

Today, at a CERN seminar, and previously at the 19^th International Conference on B-Physics at Frontier Machines, BEAUTY 2020, the LHCb collaboration announced the first observation of time-dependent CP violation in B_s⁰ decays. This discovery marks an important milestone in the history of particle physics, as shown in the time chart below.

In a fascinating world of quantum mechanics the B⁰ and B_s⁰ mesons turn into their antiparticles and back. This feature is called mixing or oscillations. The B_s⁰ mesons oscillate with an extremely high frequency of about 3 million million times per second (3*10¹²), on average about 9 times during their lifetime. This oscillation feature gives rise to a phenomenon called time dependent CP violation shown by an artist in the image as an oscillating pendulum in front of CP violating mirror.

The analysis presented at the conference reported the measurement of time-dependent CP asymmetries in B⁰→π⁺π^- and B_s⁰→K⁺K^- decays using a data sample of pp collisions corresponding to an integrated luminosity of 1.9fb⁻¹, collected at the centre-of-mass energy of 13TeV. The same data sample is used to measure the time-integrated CP asymmetries in B⁰→K⁺π^- and B_s⁰→π⁺K^- decays. The results are consistent with earlier measurements, and a combination with previous LHCb Run 1 measurements is also performed. The result of the combination constitutes the first observation (at more than 5σ) of time-dependent CP violation in B_s⁰ decays.

The left side image above shows the CP asymmetry of the B_s⁰ meson decay into K⁺K^- pairs as a function of the B_s⁰ meson decay time. An oscillating non-zero asymmetry is clearly visible, indicating the presence of time-dependent CP violation in B_s⁰ decays. The right side image above shows the analogous asymmetry for the B⁰ meson decaying into π⁺π^- pairs. Note that B_s⁰ meson oscillations are much faster than B⁰ meson oscillations. For this reason the CP asymmetry of the B_s⁰ decay is reported in the plot folded into one oscillation period, contrary to the B⁰ asymmetry plot.

The images above show the invariant mass distributions of K⁺π^- and K^-π⁺ mesons. B⁰ decays (white) dominate the spectrum, and the contributions of B_s⁰ decays are indicated in blue. The measurement of the difference between the rate of B_s⁰→K⁺π^- and B_s⁰→K^-π⁺ decays allowed LHCb to make the first observation the CP violation in the decays of B_s⁰ mesons already in April 2013 using the data sample collected in 2011. LHCb physicists simply realised that the number of B_s⁰ meson decays into K⁺π^- and K^-π⁺ pairs is different (so called time-integrated CP violation). This difference can also be observed with new data by eye in the invariant mass distributions, as a difference in the height of the two blue contributions, indicated by the horizontal line. The upper horizontal line shows that the integrated CP violation is also easily observed for the B⁰ meson decays.

The video (source) shows the dancer oscillating in front of CP violating mirror. In a given time slot the image in the mirror is different - similar to the different antimatter "image" in the B_s⁰ system.

CP violation is the mathematical description of the difference in behaviour between matter and antimatter in the Standard Model (SM) of particle physics. This difference is needed to explain the dominance of matter over antimatter that is observed in the Universe and the current description of CP violation in the SM cannot explain this very large imbalance. Hence, studies of CP violation are a promising avenue in the search for manifestations of physics beyond the SM in the data collected at the LHC. The results announced today enable improved measurements on the CKM angles α and γ and the so-called B_s⁰ mixing phase. The decays studied in this measurement are sensitive to contributions from physics beyond the SM and can be compared to the results obtained from decays where the SM processes dominate.

More details and the numerical values can be found in the conference presentation, in the CERN seminar, in the LHCb paper and also in the CERN update in English and French. You can find many other interesting results in the LHCb presentations at the BEAUTY conference web site.

11 August 2020: Observation of a strange charming tetraquark?

Today, at a CERN seminar, the LHCb collaboration announced the discovery of an exotic structure, hidden in B⁺→D⁺D^-K⁺ decays. The exotic structure, seen at a mass of 2.9 GeV/c² in the mass spectrum of the D^-K⁺ pair, could be interpreted as the first discovery of an open-charm tetraquark, or rather two, each having a different spin.

A detailed study of the beauty meson decay B⁺→D⁺D^-K⁺ allowed LHCb to make this discovery (using an amplitude analysis method). In this B⁺ decay, conventional (so-called "charmonium") resonances can only appear through their decays to a D⁺D⁻ meson pair. Should the exotic new structure be a resonance in the D^-K⁺ channel, it would have have minimal quark content cdus, as seen in the left image. The sample of 1300 B⁺ decays employed for the study is unprecedented in its purity, compared to earlier studies of B decays to two open-charm mesons, as shown in the D⁺D^-K⁺ invariant mass spectrum (right).

The B⁺→D⁺D^-K⁺ decays were studied with a help of the so-called Dalitz plot, invented in order to study particle decays into three particles. The axes of the plot are the squares of the invariant masses of two pairs of the decay products. The presence of intermediate particles -- formed for a tiny fraction of a second during the B⁺ particle decay -- is revealed through vertical or horizontal bands in what would otherwise be a uniformly-populated plot. The orientation of the band indicates the pair of particles to which the intermediate "resonance" decays, so that a charmonium resonance decaying to a D⁺D^- meson pair would produce a horizontal band. The exotic structure in the D^-K⁺ channel produces a clear vertical band. This excess cannot be accounted for by resonances only in the D⁺D^- decay channel. This is presented in today's seminar and will be explained by two forthcoming papers on this new state. The result of the analysis, presented as a blue distribution at the right image below, shows that the description with only D⁺D^- resonances is incomplete. A prominent discrepancy is apparent around a D^-K⁺ mass of 2.9 GeV/c². The significance of this structure in the amplitude model developed to describe the B⁺→D⁺D^-K⁺ decay is overwhelming. The left image below shows the D^-K⁺ invariant mass spectrum explained by contributions from different particles including the new structure, modelled according to a two-state tetraquark interpretation, X₁(2900) and X₀(2900), at a mass around 2.9GeV/c². Note the linear horizontal mass scale below and m² scale in the Daltz plot above.

In the conventional quark model, strongly interacting particles, known as hadrons, are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In their fundamental 1964 papers [1] and [2], in which they proposed the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration to form hadrons with four (tetraquark) or five (pentaquark) quark constituents. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)⁺ particle, first observed by the Belle collaboration, is composed of four quarks (ccdu). Then, a major turning point in exotic baryon spectroscopy was achieved at the Large Hadron Collider in July 2015 when, from an analysis of Run 1 data, the LHCb collaboration reported significant pentaquark structures in the J/ψp mass distribution in Λ_b⁰→ J/ψpK^- decays. One month ago LHCb physicists announced the possible discovery of a four-charm quark tetraquark.

Various interpretations of these multi-quark structures have been proposed, including tightly bound four- or five-quark states and loosely bound molecular meson-meson or baryon-meson states, as well as various other hadronic effects which can lead to evidence of exotic structure. Today’s discovery will trigger intense activity among theoretical physicists in order to understand the nature of the new D^-K⁺ structure.

Read more in the LHCb seminar, in the LHCb papers [1] and [2], in the CERN update in English and French and also in the CERN Courier article.

1 August 2020: Measurement of the CKM angle γ and study of the b→sγ photon polarisation. New interesting results presented at ICHEP 2020.

The LHCb Collaboration presented several interesting new results at the 40^th International Conference on High Energy Physics, ICHEP2020, two of which are described below.

(1) Measurement of the CKM angle γ. Within the Standard Model, CP violation can be investigated by measuring the angles and lengths of the Unitarity Triangle. The Cabibbo-Kobayashi-Maskawa (CKM) angle γ is the only CKM angle that can easily be measured with so-called tree-level processes, which can serve as a clean and precise control: the theory uncertainties associated with the measurement are very small, and even in the presence of phenomena beyond the Standard Model their effect on the measurement is expected to be small. By comparing these tree-level measurements with others that are more sensitive to physics beyond the Standard Model, scientists can test for and potentially observe new physics.

This week, LHCb physicists presented a study of the tree-level decays B^±→DK^± and B^±→Dπ^±, where D decays via D→K_S⁰π⁺π^- and D→K_S⁰K⁺K^-. The D represents here a superposition of D⁰ and D⁰ mesons, with this superposition leading to quantum mechanical interference effects that make the angle γ accessible experimentally. The analysis used the entire LHCb data set collected in Runs 1 and 2.

The presence of interference leads to differences between the Dalitz plot distributions of D decays that are reconstructed in B⁺ vs B^- decays. The images above show the DK^± and Dπ^± invariant mass distributions and those below show the typical Dalitz plot distributions used in the analysis. The experienced eye can find differences between the two Dalitz plots below. To convert a measurement of these differences into a measurement of the CKM angle γ, another input is required: knowledge of how the so-called strong phase of the D meson decay varies across the Dalitz plot. The strong-phase, which result from the interactions of hadronic final states, cannot be calculated from theory reliably and must instead be determined experimentally.

The best way to measure parameters describing the the strong phase in different regions of the Dalitz plot is with quantum-correlated decay of the ψ(3770) meson. This was done previously by the CLEO collaboration, which took data at the Cornell Electron Storage Ring (CESR) in the USA, and more recently by the BESIII collaboration, which is currently taking data at the Beijing Electron-Positron Collider II (BEPCII) in China. Uncertainties on these inputs translate to an uncertainty on γ, and with the latest results from BESIII in particular this component of the uncertainty has been significantly reduced to approximately 1°. Read more about the BESIII measurements in the phys.org article.

The value of the CKM angle γ from this new measurement, announced this week by LHCb is γ= (69±5)°. The result is limited by statistical uncertainties that will improve with additional data in Run 3, and is the most precise measurement of CKM angle γ to date from a single analysis.

(2) Study of the b→sγ photon polarisation with B⁰→K^*0e⁺e^- decays. The transition of a b-quark to an s-quark accompanied by the emission of a photon is very rare according to the Standard Model. The precise study of its properties could reveal the existence of new heavy particles. In the Standard Model this decay takes place via the W boson that carries the weak force—and since the W couples only to left-handed quarks, the photons emitted in b→sγ transitions are predominantly left-handed. Therefore, a large right-handed contribution would represent a clear sign of physics beyond the Standard Model. For example, it could represent the first sign of the existence of a heavy partner of the W boson.

To measure the photon polarisation, this analysis makes use of the b→sγ transition involving a virtual photon, which produces an electron-antielectron pair that is observed in the detector. More specifically, the decay process B⁰→K^*0e⁺e^-, which is an example of a b→s transition, is used. The largest sample of this rare decay to date is obtained using the entire pp-collision dataset collected by LHCb between 2011 and 2018. The K^*0 is reconstructed through its decay into K⁺ and π^- mesons. The polarisation of the photon is measured by an analysis of the angular distributions of the K⁺, π^-, e⁺ and e^- particles. The left image shows the invariant mass distribution of these four particles with a clear accumulation around the B⁰ mass.

Generally, decays of the form B⁰→K^*0ℓ⁺ℓ^- decay (where ℓ is an e, μ or τ lepton) can be described in the full range of q² (where q is the dilepton invariant mass) by the so-called Wilson coefficients, C^(′)₇, C^(′)₉, and C^(′)₁₀, as seen in the left image below. These coefficients encode information about the basic physical processes and are sensitive to physics beyond the Standard Model. Because the mass of a real photon is zero, the region at low q² is most sensitive to its properties. The LHCb analysis uses the very low q² range between 0.0008 and 0.257 GeV², which contains information on the C₇ and C^′₇ Wilson coefficients and which are in turn related to the b→sγ photon polarisation.

LHCb physicists announced this week that the different angular observables measured in this very low q² range are found to be in agreement with the Standard Model predictions. These new measurements allow strict constraints to be placed on the contributions to the right-handed C’₇ Wilson coefficient due to physics beyond the Standard Model, C^'₇^NP. As shown in the right-hand plot above, this week's result allow C^'₇^NP to be determined with significantly better precision than the combination of previous measurements made by Belle, BaBar and LHCb.

Read more in LHCb presentations (1) and (2) and LHCb papers (1) and (2).

1 July 2020: Observation of a four-charm-quark tetraquark. A “smoking gun” for tightly bound tetraquarks?

Today, the LHCb collaboration submitted a paper reporting the discovery of a possible tetraquark candidate, T_cccc, composed of two charm quarks and two charm antiquarks, referred to as the X(6900) in the paper. These results have been already presented at a CERN seminar on June 16^th. The full Run 1 and Run 2 data sets are used in this analysis. The invariant-mass spectrum of prompt-J/ψ pairs is investigated, where the J/ψ mesons are reconstructed through the decay J/ψ→μ⁺μ^-. The three images below show the data (black points with error bars) analysed under different assumptions as explained in the seminar and paper. A narrow peaking structure at 6900 MeV/c², matching the expected signature of the production of a new particle, and a broader structure around 6400-6600 MeV/c², close to the threshold, are observed. The top plot shows the null hypothesis of only continuum J/ψ pair production, which is found to be inconsistent with the data by more than 5σ in the mass range 6200-7400 MeV/c² in which particles composed of four charm quarks have been predicted. The decay of the new particle into the J/ψ pair suggests a minimum quark content of cccc, consistent with a T_cccc tetraquark interpretation.

This is the first observation of this unusual combination of heavy quarks. Indeed all the exotic hadrons observed so far have at most two heavy quarks and none of them is made of more than two quarks of the same type.

The peak at around 6900 MeV/c² closely resembles what would be expected for a T_cccc tetraquark particle, while the threshold bump could be due to another T_cccc tetraquark, a combination of several overlapping tetraquark peaks, or a more complicated decay of a T_cccc in which additional final-state particles were created beyond the two J/ψ that were reconstructed.

A special effort was made to enhance the contribution, in the J/ψ-pair invariant-mass spectrum, of events that could potentially contain tetraquarks with respect to those that could not. Prompt J/ψ pairs can be produced both through double-parton (quark or gluon) scatterings (DPS), where the two J/ψ mesons are produced independently in separate partonic interactions, and single parton scattering (SPS), which includes non-resonant and T_cccc production. Specific criteria, explained in the paper, were applied in the analysis in order to enhance the selection of SPS over the DPS events in the final sample.

In the conventional quark model, strongly interacting particles, known as hadrons, are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In their fundamental 1964 papers [1] and [2], in which they proposed the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration to form hadrons with four (tetraquark) or five (pentaquark) quark constituents. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)⁺ particle, first observed by the Belle collaboration, is composed of four quarks (ccdu). Then, a major turning point in exotic baryon spectroscopy was achieved at the Large Hadron Collider in July 2015 when, from an analysis of Run 1 data, the LHCb collaboration reported significant pentaquark structures in the J/ψp mass distribution in Λ_b⁰→ J/ψpK^- decays.

Various interpretations of these multi-quark structures have been proposed, including tightly bound four- or five-quark states and loosely bound molecular meson-meson or baryon-meson states. Which possibility is realized in nature? Well, perhaps both. The LHCb results presented two weeks ago are a clear example of this puzzle, as the χ_c1(3872) particle could be interpreted as a quasi-bound D⁰D^*0 molecule, being a tightly bound four-quark state also a possible explanation of its nature. The newly observed T_cccc tetraquark represents the first observation of a highly unusual combination of four charm quarks. Unraveling the mystery of the nature of this new particle will provide an exceptional laboratory to test and constrain many theoretical models. On the one hand, the observed mass peak could represent a “smoking gun” proof for the existence of tightly bound four-quark systems possibly composed of a diquark and an anti-diquark mediated by a short-range qluon exchange, as shown in the image. This interpretation may be hinted by the fact that the mass of the particle seen is located above the masses of known charmonium cc and exotic X, Y and Z particles, as well as below the bottomonium bb system. But, on the other hand, interpretation as a di-meson molecular state, or the possibility that the observed peak hides a more complex underlying structure are also well plausible.

Today’s discovery will trigger additional intensive activity among theoretical physicists. The LHCb collaboration continues the search for new, not yet discovered, exotic particles and the study of their properties.

Read more in the LHCb CERN seminar presentation and paper, in the CERN update in English and French and also in the CERN Courier article.

30 June 2020: Chris Parkes and Matteo Palutan – new management for the LHCb Collaboration

Chris Parkes from the University of Manchester begins tomorrow his three-year tenure as LHCb spokesperson. He takes over from Giovanni Passaleva from the Istituto Nazionale di Fisica Nucleare in Firenze. Chris Parkes was previously the deputy spokesperson of the collaboration. Matteo Palutan from the INFN e Laboratori Nazionali di Frascati is the new deputy spokesperson.

Chris and Matteo will face the huge challenges of leading us through the completion of the major LHCb detector upgrade during Long Shutdown 2, LS2, and preparing the data-taking with a practically new detector and the new fully software trigger system. They dream that the analysis of the full Run 1 and Run 2 data samples, and the first samples from Run 3, could yield the discovery of new physics and many new particles! They are keen to continue the wonderful friendly spirit of open collaboration and support for all that is a hallmark of our experiment.

Giovanni and Chris - thank you for your excellent coordination in the past three years.

Chris and Matteo – good luck.

Read more details at the LHCb collaboration web page and CERN update in English and soon in French.

28 May 2020: More on χ_c1(3872) puzzle.

Today, the LHCb collaboration submitted two papers reporting precise measurements of the properties of a mysterious particle χ_c1(3872), also known as the X(3872). These results have also been presented at the conference on Large Hadron Collider Physics, LHCP. Since its discovery 17 years ago physicists try to understand if this particle is a conventional charmonium state composed of c and c quarks or if it is an exotic particle composed of four quarks. An exotic particle of this type could be a tetraquark, a molecular state, a cc gluon hybrid state, a vector glueball or a mixture of different possibilities. A very interesting feature of this particle is that its mass is remarkably close to the sum of the masses of the D⁰ and D^*0 mesons.

The particle was originally discovered in B⁺ meson decays, specifically the process B⁺→X(3872)K⁺, by the BELLE collaboration in 2003. Its existence was later confirmed by the CDF, D0 and BaBar experiments. LHCb first reported studies of the X(3872) in the data sample taken in 2010 (see 27 October 2010 news) and later unambiguously determined its quantum numbers to be 1⁺⁺. This quantum number determination lead the Particle Data Group to change the name of this particle to χ_c1(3872). Despite studies of the properties of this particle over 17 years by different experiments at different accelerators its nature is still not definitely understood.

Two different, minimally overlapping data sets were used. First, a Run 1 data set corresponding to an integrated luminosity of 3 fb^-1 of data in which about 15 500 χ_c1(3872) particles were selected from decays of hadrons containing b quarks. Second, the full Run 1 and Run 2 data set corresponding to an integrated luminosity of 9 fb^-1 in which about 4 230 B⁺→χ_c1(3872)K⁺ decays were selected. In both cases the χ_c1(3872) particles were reconstructed in their J/ψπ⁺π⁻ decay mode. The image shows the accumulation of events around an invariant mass of 3872 MeV in the second data set.

Precise measurements of its parameters, namely the mass and the natural width, are crucial to understanding the nature of χ_c1(3872). LHCb has announced today the first measurement of the width Γ of this particle. Up to now only the upper limits were reported by different experiments. The width of the particle is related to its lifetime by the basic rules of quantum mechanics. The fact that the mass of the χ_c1(3872) is close to the sum of the masses of the D⁰ and D^*0 mesons makes this analysis particularly challenging. Two models were used to study the χ_c1(3872) lineshape (Breit−Wigner and Flatté, for experts). The decay D⁰D^*0 was taken explicitly into account in addition to the J/ψππ decay. The presence of this decay distorts the shape of the signal. This distortion needs to be taken into consideration if one wants to measure the physical mass and the width of the χ_c1(3872). The left image below shows a summary of the current limits (arrows) and measurements of the χ_c1(3872) natural width performed by different experiments. The two LHCb entries (red points) correspond to the two LHCb results announced today.

The right image below shows a summary of measurement of the χ_c1(3872) mass performed by different experiments. The LHCb results (red points) are the most precise. In each plot, the orange band represents the value and the uncertainty on the world average including new LHCb results.

LHCb physicists announced also today that the χ_c1(3872) mass coincides with the sum of the masses of the D⁰ and D^*0 mesons - the measured difference is only 70±120 keV, see corresponding entry (blue point) in the right plot above.

To understand the nature of the χ_c1(3872) it is hepful to compare its properties to those of conventional charmonium particles with similar mass. LHCb physicists compared the production of the χ_c1(3872) with the production of ψ₂(3823) and ψ(2S) mesons accompanied by a K⁺ meson in B⁺ decays. The decay ψ₂(3823)→ J/ψπ⁺π^- is observed for the first time with significance exceeding 5 standard deviations.

These new results from LHCb add new pieces to the χ_c1(3872) puzzle. The results favour the interpretation of the state as a quasi-bound D⁰D^*0 molecule, however, no final solution has been reached yet. LHCb physicists continue to work on this subject, more results will come in the future and will be reported on this page. The next planned analysis will employ a sophisticated, state-of-the-art amplitude analysis technique to study the χ_c1(3872) particle. This approach is discussed in detail in one of the papers released today, and is ilustrated by the multicoloured plot shown above.

Details and numerical values can be found in the LHCb papers [1] and [2]. Read more in the CERN Courier article, in the CERN LHCP media update in English and French, and also in LHCb LHCP presentations [1] and [2].

9 April 2020: High Energy physics community ventilator combatting the COVID-19 pandemic.

CERN has established a task force to identify and support contributions from the Organization’s 18 000-strong global community to combat the COVID-19 pandemic.

A team of physicists, engineers and technicians from the LHCb collaboration, supported by several LHCb institutions and CERN services, has developed a novel streamlined ventilator, called HEV. As the pandemic spreads, the number of hospitalised patients requiring ventilators has led to a global shortage of supplies. The team realised that the types of systems used to regulate gas flows for particle physics detectors could be used to design a novel ventilator. The HEV design could be used for patients in mild or recovery phases, enabling the more high-end machines to be freed up for the most intensive cases. It is a safety-first design, intended to satisfy clinical requirements for the most requested ventilation modes for COVID-19 patients.

Learn more in the CERN news, in the CERN Courier article, in the LHCb video, and in the original publication.

31 March 2020: Observation of new Ξ_c⁰ baryons.

Today, the LHCb collaboration submitted a paper reporting a system of three particles interpreted as three narrow excited Ξ_c⁰ - states. The Ξ_c⁰ baryon is composed of a charm-, a strange- and a down-quark (csd).

The lightest of all baryons, the proton, which is the nucleus of the hydrogen atom, is composed of two up- and one down-quark (uud) while its neutral partner the neutron is composed of two down- and one up-quark (ddu). If one (or more) light quark is replaced by either a charm c or a beauty b heavy quark we obtain heavier charmed or beauty baryon particles. The three quarks can also be formed in their lowest-energy quantum mechanical state: the ground state. Like electrons in atoms, the quarks can be rearranged into excited states with different values of angular momentum and quark spin orientation. Following particle physics tradition these excited states are called particles.

LHCb physicists searched for excited Ξ_c⁰ states in their decay into a Λ_c⁺ baryon and a K^- meson. The Λ_c⁺ baryons were reconstructed in their decays into pK^-π⁺. The image shows the distributions of the Λ_c⁺ K^- invariant mass minus the Λ_c⁺ and the K^- masses. Three narrow structures are clearly visible. They represent an observation, with large statistical significance, of three excited states of the Ξ_c⁰ baryon. They are named Ξ_c(2923)⁰, Ξ_c(2939)⁰ and Ξ_c(2965)⁰. The numbers in brackets represent the masses of these states as measured by LHCb.

While other excited Ξ_c states have been reported before, the Ξ_c(2923)⁰ and Ξ_c(2939)⁰ are observed as new narrow states for the first time. The Ξ_c(2965)⁰ seen by LHCb is close to a previously known state, the Ξ_c(2970)⁰, but its measured mass and width properties differ significantly from the average of existing Belle and BABAR results for the Ξ_c(2970)⁰. Further studies will be required to determine whether more than one state is present in the 2965-2970 MeV region, and, if so, the relation between them.

Today’s discovery of excited states of the Ξ_c⁰ baryon, together with previous discoveries of other excited states of baryons containing heavy quarks (such as excited Λ_b, Σ_b, Ω_c and Ω_b), will improve our knowledge of the quantum structure of charmed and beautiful baryons. Read more in the LHCb paper and in the APS Synopsis.

10 March 2020: B⁰→K^*μ⁺μ^-: more data confirm old puzzle.

Today at the CERN seminar, the LHCb collaboration presented an analysis of the angular distributions of B⁰→K^*μ⁺μ^- decays. The measurement uses the same technique as a previous measurement performed on the 2011 and 2012 data sample, but twice as many B⁰ decays were analysed, due to the addition of data taken during 2016. Compared with the previous LHCb results, the overall tension with the Standard Model (SM) is observed to mildly increase.

The analysis of B⁰→K^*μ⁺μ^- decays is considered to be a very promising way to search for effects of as-yet-undiscovered particles (see the CERN Courier article for an introduction). At the quark level this decay involves the transition from a b-quark to an s-quark, accompanied by the production of a muon pair (μ⁺μ^-). The plots show the recontructed B⁰ mass for the Run 1 data (left) and 2016 data (right). The decays are sensitive to virtual contributions from new particles, which could have masses that are inaccessible to direct searches at the LHC. The reported tension with the SM predictions can be explained consistently in some new physics models. However, the analysis of these decays is complicated; the best sensitivity to new particles comes from the study of the angular distribution of the kaon and pion from the K^* decay and the muons. Physicists from the LHCb experiment have studied different angular observables as functions of the invariant mass squared, q², of the muon pair. A set of observables are analysed whose theoretical predictions are much less dependent on a good understanding of the hadronic physics involved in turning a B meson into a K^* meson (so-called form factors). These observables are therefore ideal for searching for new particles in these decays. It is one of these observables, "P₅^'", that showed a local deviation with respect to the SM predictions at a level of 3.7 standard deviations (σ) in one q² bin in the 2011, 1fb^-1 data sample presented at the 2013 EPSHEP Stockholm conference and published soon after. The global fit of different angular distributions of the B⁰→K^*μμ decays was performed in 2015 using the total Run 1 data sample of 3fb^-1 and confirmed the puzzle, reporting differences with predictions based on the SM at the level of 3.4σ. These differences could be explained by contributions from physics beyond the SM, or by unexpectedly large hadronic effects that are not accounted for in the predictions. By taking into account some of these hadronic effects, and other effects, using theory input not available at the time of the 2015 publication, the tension with respect to the SM is reduced to 3.0σ.

The image shows the distribution of the P₅^’ observable as a functions of the mass squared of the muon pair, q². The q² regions affected by the presence of the J/ψ and ψ(2S) charmonium resonances, where theoretical predictions are prohibitively difficult to obtain, are not included in the analysis. The red points show the LHCb Run 1 results, the blue points represent the results of the analysis using the additional 2016 data set only, while the black points display the results of the simultaneous analysis of the both sets of data. The SM predictions are presented as orange boxes. These were taken from calculations described in theoretical papers [1] and [2].

The local tension of the P₅^’ observable in the 4.0q²6.0 and 6.0q²8.0GeV²/c⁴ bins reduces from the 2.8 and 3.0σ, observed in the Run 1 analysis, to 2.5 and 2.9σ in the simultaneous analysis. However, the global fit to several angular observables shows that the overall tension with the SM increases from 3.0 to 3.3σ. The global fit uses not only the P₅^’ distribution but also a few other distributions mentioned in the 9 August 2013 news. The results of the fit find a better overall agreement with predictions of a certain new physics model. (For experts: the data could be explained by modifying the real part of the vector coupling strength of the decays, conventionally denoted Re(C₉).) Data-SM comparisons for the full set of observables are given in the paper.

LHCb physicists are systematically studying a principle of the SM of particle physics known as "lepton universality", which states that the laws of physics treat the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses. Different ratios are investigated that compare beauty particle decays to different leptons (see R_K, R_K^*0, R(D^*), R(J/ψ) and recently R_pK). These measurements revealed hints of deviations from lepton universality, none of which is significant enough to constitute evidence of new physics independently. However, according to theorists who study possible extensions of the SM, taken together these deviations suggest an interesting and coherent pattern. Is there anything common between lepton universality anomalies and angular distribution anomalies reported today? Well, maybe! According to theorists both types of anomalies can be coherently described by the same new physics models.

Read more in the LHCb CERN presentation, in the LHCb paper, in the CERN Courier article and also in the CERN update in English and French.

22 January 2020: Observation of a new beauty baryon particle.

Today at the Winter Meeting on Nuclear Physics, Bormio, Italy, the LHCb collaboration announced the discovery of a new beauty baryon particle. The new particle is observed in the Λ_b⁰π⁺π^- spectrum using the full LHCb Run 1 and Run 2 data set, corresponding to an integrated luminosity of 9fb^-1.

The lightest baryon, the proton, which is the nucleus of the hydrogen atom, is composed of three light quarks uud while its neutral partner the neutron is composed of udd quarks. If one of the d quarks is replaced by a heavier strange quark s, we obtain a Λ⁰ particle composed of uds quarks. Furthermore, if the s quark in the Λ⁰ baryon is replaced by a charm quark c or a beauty quark, we obtain a Λ_c⁺ or a Λ_b⁰ baryon particle. The three quarks udb forming the Λ_b⁰ are in their lowest-energy quantum mechanical state: the ground state. Like electrons in atoms, the quarks can be rearranged into excited states with different values of angular momentum and quark spin orientation. LHCb has already announced the observation of the two excited states of Λ_b⁰ baryon, known as the Λ_b(5912)⁰ and Λ_b(5920)⁰, in 2012; another discovery followed more recently, with the existence of two additional excited states, Λ_b(6146)⁰ and Λ_b(6152)⁰, being announced six months ago. All these particles were discovered in the Λ_b⁰π⁺π^- invariant mass spectrum. The first two states are interpreted as the first orbital excitations of the Λ_b⁰ baryon, while the other two are compatible with predictions for the 1D excited states. The particle announced today, Λ_b^**0, adds another brush stroke to the painting and is interpreted as the first radial excitation, the 2S state.

The first stage of the analysis is to reconstruct the Λ_b⁰ ground state. These baryons are formed in two different ways: from Λ_c⁺π^- and J/ψpK^- combinations, with J/ψ→μ⁺μ^- and Λ_c⁺→pK^-π⁺. The images at the left show the invariant mass spectra of the large, low-background samples of reconstructed Λ_b⁰ baryons. The images below show the Λ_b⁰π⁺π^- invariant mass spectrum, with data points obtained using the two independent Λ_b⁰ decay modes.

The two narrow peaks in blue and green represent the two excited states Λ_b(6146)⁰ and Λ_b(6152)⁰ discovered by LHCb six months ago. The broad red distribution shows the contribution from the Λ_b^**0 particle whose discovery was announced today. The broad Λ_b^**0 peak has an overwhelming statistical significance for each Λ_b⁰ decay mode individually, well above the threshold necessary to claim a discovery. The broad width indicates that the 2S excited state has a shorter lifetime than the narrow 1D states, according to the fundamental rules of quantum mechanics. The mass of the new Λ_b^**0 baryon is measured to be 6072.3±2.9±0.6±0.2 MeV, and its width Γ is 72±11±2 MeV.

Earlier this year, the LHCb collaboration reported a new system of particles interpreted as four narrow excited states of Ω_b^- baryon (with two above the observation threshold). In 2018 two excited states of Σ_b baryons were observed by LHCb. With these and other LHCb observations the total number of known beauty baryons has more than doubled. LHCb physicists have also discovered new charmed baryons such as five excited states of the Ω_c⁰ in 2017.

In the 19^th century, physicists observed that different elements emitted light in narrow coloured spectral lines. At the beginning of 20^th century, quantum mechanics explained these spectral lines as manifestation of transitions from excited states of atoms. In the 1950s, physicists studied excited states of atomic nuclei where transitions from excited states were observed as emission of energetic gamma rays or other particles. Today, LHCb physicists continue work of their predecessors by studying excited states of quark systems and discovering the rich spectrum of Λ_b⁰ and Σ_b⁰ resonances. Here the transitions are not observed as electromagnetic radiation, but as emission of pions (π) or pion pairs (π⁺π^-).

Read more in the LHCb conference presentation and in the LHCb paper. In addition, a CMS paper appeared on the arXiv today, confirming the previously reported LHCb observations of four narrow excited states of the Λ_b⁰ baryon in the Λ_b⁰π⁺π^- invariant mass spectrum. CMS also reported evidence for a broad excess of events in the region 6040–6100 MeV, with mass and width consistent with those of the Λ_b^**0 just observed by LHCb.

6 January 2020: Beautiful charming and magic Ω baryons. First observation of excited Ω_b^- states.

Today, the LHCb collaboration submitted a paper reporting a new system of four particles interpreted as four narrow excited Ω_b^- states.

As in many particle physics analyses, the LHCb physicists had to sift through a large amount of data, piecing together particles detected by their experiment (in this case protons, kaons, and pions) and looking for the signature left by the decay of a heavier particle. They first reconstructed a highly pure sample of beauty baryons Ξ_b⁰ via their decays into a charmed baryon Ξ_c⁺ and a pion π^- as seen in the left image below. The red peaking distribution shows the Ξ_b⁰ contribution above the background indicated by the green dashed line. The Ξ_c⁺ baryons were identified through their decays into a proton p, a kaon K^- and a pion π⁺.

The next step was to combine the Ξ_b⁰ baryons with K^- mesons present in the same event. The difference between the mass of the Ξ_b⁰K^- pair and the mass of the Ξ_b⁰ alone is shown in the right image above, revealing four narrow structures. They correspond to Ξ_b⁰K^- masses significantly heavier than the ground state of the Ω_b^- baryon, which was discovered a decade ago at the Tevatron. The discoveries announced today are interpreted as excited states of the Ω_b^- baryon. These states are named the Ω_b(6316)^-, Ω_b(6330)^-, Ω_b(6340)^- and Ω_b(6350)^-, where the numbers indicate their approximate masses in MeV as measured by LHCb. These narrow peaking structures have local significances ranging from 3.6 to 7.2 standard deviations.

LHCb physicists discovered in 2017 in a similar way five excited states of the Ω_c⁰ baryon in the Ξ_c⁺K^- invariant mass spectrum, shown in the image to the left.

The Ω_b^- and Ω_c⁰ baryons are higher mass partners of the Ω^- baryon, a particle that played a very important role in the history of particle physics. In the 1950s, many different particles were discovered. While initially thought to be elementary, the ever-growing list of discoveries led physicists to doubt this assumption. Therefore efforts were made to find a classification scheme in analogy to the periodic table of chemical elements. The most successful such scheme was proposed by Gell-Mann. In this model, mesons and spin ¹⁄₂ baryons are organized into octets (referred to as the Eightfold Way), while spin ³⁄₂ baryons form a decuplet, as displayed in the left image below. When the scheme was proposed, the top-most particle in the image, the Ω^-, was not yet discovered, but hypothesised to exist in order to complete the pattern. The regular structure of the decuplet enabled many properties of this new particle to be predicted, including its mass. The Ω^- was subsequently discovered through a single famous bubble-chamber photograph obtained at the Brookhaven laboratory in 1964 (the corresponding line diagram is shown in the right picture below). This picture validated the Eightfold Way, and led Gell-Mann to propose the quark model in 1964, which explains the structure of the octets and decuplet. In the quark model the Ω^- is composed of three strange quarks (sss). The Ω_c⁰ and Ω_b^- have a similar structure to the Ω^-, but contain a charm (c) or a beauty (b) quark in place of one of the strange quarks (Ω_c⁰, css; Ω_b^-, bss).

By discovering the Ω_b^- and Ω_c⁰ excited state systems, LHCb has reinforced the exceptional role of the Ω baryon family in the history of particle physics. The discovery of these systems will provide important experimental input to constrain models used to describe the structure of baryons containing heavy flavor. In particular, a number of different theories were proposed to explain the five narrow Ω_c⁰ excitations, including models of molecular and pentaquark states. These theories can now be tested by comparing their predictions for the Ω_b^- system against these new LHCb measurements.

Read more in the LHCb paper and in the CERN Courier article.

18 December 2019: Test of lepton universality with beauty baryons, R_pK.

Today the LHCb Collaboration submitted for publication a measurement of the ratio R_pK. This tests a principle of the Standard Model of particle physics known as "lepton universality", which states that the laws of physics treat the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses.

The ratio R_pK describes how often a Λ_b⁰ baryon decays to a proton and a charged kaon and either a muon and anti-muon pair (pK^-μ⁺μ^-) or an electron and anti-electron pair (pK^-e⁺e^-). The decays involve the transformation of a beauty quark into a strange quark (b→s) plus a lepton pair (μ⁺μ^- or e⁺e^-), a process that is highly suppressed in the Standard Model but which could be affected by the existence of new particles, whose masses could be too high to be produced directly at the Large Hadron Collider.

LHCb has studied a number of other such ratios that compare beauty particle decays to different leptons (see R_K, R_K^*0, R(D^*) and R(J/ψ)). These results revealed hints of deviations from lepton universality, none of which was significant enough to constitute evidence of new physics. However, according to theorists who study possible extensions of the Standard Model, taken together these deviations suggest an interesting and coherent pattern. The measurements use the technique of blind analysis, in which the physicists analysing the data do not know the result until the analysis method is finalized and frozen, following an extended review within the collaboration.

To minimise the influence of detector and other experimental effects, LHCb physicists used a "double ratio" method in which R_pK is divided by another ratio, r_J/ψ. The true value of r_J/ψ is known to be very close to 1, but it would be influenced by detector effects in a similar way to R_pK. This gives an extra layer of protection: the scientists study and correct for all known experimental effects, but if any unknown effects slip through they would cancel in the double ratio between R_pK and r_J/ψ. In more detail, R_pK is defined as the ratio of probabilities that a Λ_b⁰ decays to pK^-μ⁺μ^- or pK^-e⁺e^- (within particular mass ranges for the pK and μ⁺μ^- or e⁺e^- pairs; see below), and r_J/ψ is defined as the ratio of probabilities that a Λ_b⁰ decays to J/ψpK^- with J/ψ→μ⁺μ^- vs e⁺e^-. The double ratio method greatly reduces systematic uncertainties related to the different experimental treatment of muons and electrons, which largely cancel in the double ratio. In particular, the use of the double ratio method means that the detection efficiency of the decay Λ_b⁰→pK^-e⁺e^- only needs to be known relative to that of Λ_b⁰→J/ψ(→e⁺e^-)pK^-, rather than Λ_b⁰→pK^-μ⁺μ^-. Moreover, J/ψ meson decays into μ⁺μ^- and e⁺e^- pairs are known to respect lepton universality at the 0.4% level.

To make the statistical analysis simpler, the main result reported in today’s paper is in fact the inverse of R_pK, ie. R^-1_pK; this just means that the ratios of decay probabilities are computed by dividing the modes with an e⁺e^- by the modes with a μ⁺μ^-, rather than the other way around. This puts the smaller electron yields in the numerator, making the uncertainty of the final result more symmetrical.

The figures show the measured invariant mass distributions of Λ_b⁰ baryons for the four decay modes used in the double-ratio measurement, each with a clear peak around the Λ_b⁰ baryon mass.

The analysis is performed in the range 0.1q²6.0 GeV²/c⁴, where q² is the invariant mass squared of the μ⁺μ^- or e⁺e^- pair, and requiring the pK^- mass to be less than 2.6 GeV/c², using proton-proton collision data corresponding to an integrated luminosity of 4.7 fb^-1 recorded at center-of-mass energies of 7, 8 and 13TeV. The value of R^-1_pK is measured to be 1.17^+0.18_-0.16±0.07, where the first uncertainty is statistical and the second systematic. For comparison with other lepton universality tests, R_pK is computed to be R_pK=0.86^+0.14_-0.11±0.05, which is compatible with 1 within one standard deviation. It is also in agreement with previous LHCb results in the lepton universality tests with B mesons, R_K and R_K*, which found hints that decays with μ⁺μ^- pairs occurr at lower rates than those with e⁺e^- pairs (i.e. R1). The results of R^-1_pK measurement presented here show the same tendency, though substantially more data will be needed to confirm or exclude a possible New Physics interpretation. This first test of lepton universality with B baryons was possible thanks to the first observation of the Λ_b⁰→pK^-e⁺e^- decay. It is also worth noting that the current analysis is sensitive to different experimental uncertainties from the previous lepton universality tests performed with B mesons, and therefore provides a complementary test of the Standard Model.

Read more in the LHCb paper, in the CERN Courier article and also in the CERN Update in English and French..

5 December 2019: LHCb Industry Awards.

The LHC Run 2 ended a year ago. The LHCb experiment has been largely dismantled and an almost completely new detector is under construction in these very days: a new LHCb will be born in 2021. Almost all LHCb's sub-detectors will be replaced or upgraded during the ongoing LHC shutdown and the new upgraded detectors will profit from the most recent technological developments. LHCb physicists and engineers have been assisted in their effort by excellent companies that have collaborated closely with LHCb and their contributions were crucial to ensure the successful upgrade of the experiment. The LHCb collaboration selected this year four outstanding companies, 3D Systems, Automation NV, LETI-3S and SOMACIS for the LHCb Industry Awards, and presented awards to these at a special ceremony which took place during the plenary collaboration meeting taking place this week at CERN. The images below show the awarded industry teams together with LHCb spokesperson and technical coordinator as well as LHCb physicists and engineers collaborating directly in the corresponding projects - the Kuraray team received this year the 2018 Award.

3D Systems produced the thin 3d-printed titanium bars with embedded cooling channels for the SciFi Tracker of the LHCb Detector. The challenge was to realize cooling pipes with a very complex shape in a tight space, very thin walls for thermal conductance combined with very tight mechanical tolerances (50 microns flatness), divided in subsections to minimize the effect of different thermal expansion coefficients and having an effective gluing surface. The scintillating-fibre tracker, SciFi, will be placed behind the dipole magnet of LHCb. The scintillating fibres emit light when a particle interacts with them and the tiny amount of emitted light is collected by silicon photomultipliers (SiPM) that convert it to electrical signals. The SiPMs are operated at low temperatures, about -40°C, and are therefore attached to the cooling bars as seen in the image.

Automation NV is responsible for the design, manufacturing, and installation of the new LHCb Data Centre (image). The company developed an innovative and very cost-effective cooling system based on a combination of adiabatic and indirect free-air cooling. An important part of the project is the cable management with 19 000 long-distance fibres and hundreds of inter-module connections. The new Data Centre will allow LHCb to implement a novel scheme of the data acquisition system. During Run 1 and 2 the event rate was filtered down to 1 MHz with the help of fast electronics, using comparatively simple algorithms to select the most interesting events. Those events were then processed in a dedicated computer farm, located underground close to the detector; this allowed additional, more sophisticated selection criteria to be applied using software. For Run 3 and beyond, this will change radically: the fast electronics will be removed and the whole detector will be read out at the full LHC pp collision rate of 40 MHz. This will allow the whole selection to be done in software in the new Data Centre located at the surface, meaning that it can be much more precise and flexible.

The team at Leti-3S developed new techniques to produce large silicon cooling plates with bi-phase CO₂ circulating into micro-channels fabricated inside the plates themselves. The plates, just half a millimeter thick, will be used to cool the hybrid pixel sensors of the new LHCb VELO (Vertex Locator) detector. Leti developed a complex multistep microfabrication process, deploying supreme control of the channel dimensions, the strength of the wafer bonding, and the precise etching of the final pieces. The devices were produced on 8-inch silicon wafers, shown in the image, featuring two micro-channel cooling plates and smaller test structures. The new VELO will come as close as 5 mm to the proton beams and therefore will allow LHCb to localise precisely the points at which beauty and charm particles decay (read more and more).

SOMACIS produced Printed Circuit Boards (PCB) for the large and sophisticated data acquisition electronic boards PCIe40. The PCBs (image) are complicated due to a large number of components being interconnected. Some of them have a very dense matrix of connections over the 14 layers of PCBs. Layers are not symmetric, 35 µm of copper on one side and 70 µm on the other one. Some planes carry current up to 60 A with high speed signal up to 10 Gbit/s. The PCIe40 boards are essential elements of the new LHCb data acquisition system. The boards are located in the Data Centre and receive data through optical fibre links transmitting signals from the underground detector to the Data Centre at the surface at the 40 MHz rate. The boards are programmable and perform different tasks for different sub-detectors preparing data for further analysis in the computer farm.

15 November 2019: Celebrating LHCb's 500^th publication.

The LHCb collaboration has submitted its 500^th publication! The first LHCb paper was submitted in August 2010, and since then 50 to 70 papers were submitted per year. All LHCb papers are accessible on the arXiv and are published Open Access.

The 500^th paper reports sophisticated, multidimensional analysis through which the quantum numbers of several excited charmed mesons are determined. The charmed mesons were studied through the patterns they produce in certain decays of beauty mesons: B^-→D^*+π^-π^-. The so-called Dalitz plot (image) and an analysis of angular distributions were used to measure the quantum numbers of several excited charmed mesons that can decay into a D^*+π^- pair.

14 November 2019: Production of exotic meson candidate χ_c1(3872) in high-multiplicity events and other interesting results presented at the Quark Matter conference.

The LHCb Collaboration presented several new interesting results at the Quark Matter 2019 conference in Wuhan, China. Among them was a study of how the production rate of the exotic meson candidate χ_c1(3872) (historically known as the X(3872)) in pp collisions depends on the multiplicity - the number of particles in the event.

In the conventional quark model, strongly interacting particles known as the hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons, or antibaryons for three antiquarks). Particles that interact strongly but cannot be classified within this scheme are referred to as exotic hadrons. LHCb discovered particles composed of four quarks and an antiquark (pentaquarks) and made essential contribution to study of particles composed of two quarks and two antiquarks (tetraquarks). The χ_c1(3872) was first discovered in the mass spectrum of J/ψπ⁺π^- in B-hadron decays at Belle, and has since been confirmed in multiple decay modes at other experiments including LHCb. Multiple explanations of the χ_c1(3872) structure have been proposed. Shortly after its discovery, it was considered as one of several possible charmonium (cc) states. However, LHCb measurements have since confirmed the quantum numbers to be J^PC = 1⁺⁺, which disfavours an assignment as a conventional charmonium particle because no compatible charmonium states are expected to exist near the measured mass. The remarkable proximity of the χ_c1(3872) mass to the sum of the masses of the D⁰ and D^*0 mesons led to the consideration that its structure might be a hadronic molecule, i.e. a state consisting of these two mesons loosely bound together. In this case, the binding energy of the χ_c1(3872) hadron would be small and its radius large.

Weakly bound quarkonia (cc, bb) states have been studied extensively in proton-nucleus (pA) and nucleus-nucleus (AA) collisions. Measurements of charmonia production in pA collisions showed that the ψ(2S) is suppressed more than the J/ψ in rapidity regions where a relatively large number of charged particles are produced; this is expected since the ψ(2S) is more loosely bound than the J/ψ. If the χ_c1(3872) is a hadronic molecule, similar effects could also disrupt its formation in dense environments. The LHC has produced large samples of pp collisions, including high-multiplicity pp collisions that provide a hadronic environment that approaches heavy-ion collisions in many respects. Recently, phenomena typically thought only to occur in collisions of large nuclei have been observed in high-multiplicity pp collisions. Thus, if the χ_c1(3872) is a hadronic molecule, suppression effects might also be seen in high-multiplicity pp collisions.

The LHCb collaboration presented measurements of the fractions of χ_c1(3872) and ψ(2S) states that are produced promptly (directly at the pp collision point), f_prompt, in pp collisions at 8 TeV as a function of the event activity. The χ_c1(3872) and ψ(2S) candidates are reconstructed through their decays to J/ψπ⁺π^- as seen in the image above. The insert plot shows a zoom around χ_c1(3872) mass.

Promptly produced particles may interact with hadrons produced at the same time in the pp collision. A very important feature of the LHCb analysis is the ability to compare the promptly produced χ_c1(3872) and ψ(2S) particles with those from decays of B hadrons. This comparison is very interesting because B hadrons live long enough to fly a few mm and decay in the vacuum outside the pp collision region - and therefore the χ_c1(3872) and ψ(2S) produced in B hadron decays never interact with the dense environment at the pp collision point, and so could not experience the suppression effects mentioned above.

The left image shows the fraction f_prompt of promptly produced χ_c1(3872) and ψ(2S), as a function of the number of tracks reconstructed in the VErtex LOcator (VELO), which is a measure of the event activity. For both mesons, the value of f_prompt decreases as the event activity increases. The right image shows the ratio of the χ_c1(3872) and ψ(2S) production cross sections for prompt and B hadron decay production (denoted "b decays"), again as a function of the number of tracks reconstructed in the VELO. Moving from low to high multiplicity, the data suggest that prompt χ_c1(3872) production is suppressed relative to prompt ψ(2S) production. This would be expected in a scenario where interactions with co-moving hadrons produced in the collision dissociate a large, weakly bound χ_c1(3872) particle more than the relatively compact conventional charmonium ψ(2S) particle. In contrast, the ratio of cross sections for production in B hadron decays does not display any significant dependence on event activity. (The central values of the black points increase gradually, but within uncertainties are consistent with being flat.) These results will be of interest both to theorists studying the possible exotic nature of χ_c1(3872) particle and those investigating dense hadronic matter. Read more in the conference presentation and in the conference note.

Results from LHCb on Open and hidden beauty production as well as Z boson production in pPb and Pbp collisions were also presented at the conference. These allow the study of cold nuclear matter effects, and to how they can be disentangled these from quark-gluon plasma effects (see an introduction to the topic). The images above show the production of different bb quark bound states (ϒ mesons) and Z boson production in pPb collisions as well as production of two charmonium states (J/ψ and ψ(2S)) in ultra-peripheral Pb-Pb collisions. The LHCb Upgrade will provide an excellent opportunity for unique and groundbreaking insights on the intrinsic properties of the heavy ion physics to be made in the LHC Run 3 and 4.

16 October 2019: More and more precise measurements.

The LHCb Collaboration presented interesting new results at the International Conference on Kaon Physics 2019 in Perugia, Italy, and at the International Conference on B-physics at frontier machines, Beauty 2019 in Ljubljana, Slovenia. Selected topics are listed below.

(1) Hunting for the rarest strange decays at the LHC, K_S⁰→μμ. Decays of K mesons into pairs of muons played a very important role in the history of particle physics. There are two types of neutral K mesons: the short-lived K_S⁰ ("K-short"), and the long-lived K_L⁰ ("K-long"). The results of branching-ratio measurements of the K-long decay into muon pairs in the early 1970s disagreed strongly with the predictions of the particle physics theory of the time, based on the existence of three quarks: u, d and s. The branching ratios were calculated to be of the order of 10^-4 while the experimental limits were about 4 orders of magnitude lower. In an attempt to solve this problem Glashow, Iliopoulos and Maiani proposed the existence of an additional quark, called a charm quark – a 1970s version of a new physics model. In the mechanism they proposed (GIM mechanism) a destructive virtual contribution of this new quark greatly reduced the K-long decay rate into muon pairs. The discovery of the J/ψ meson in November 1974 was the first evidence for the existence of charm quark, and at the same time confirmation of the GIM mechanism.

45 years later, LHCb physicists are searching for K-short decays into muon pairs, again on the look-out for new physics. This decay rate is very sensitive to possible contributions from new, yet-to-be discovered particles, such as leptoquarks or super-symmetric particles that are too heavy to be observed directly at the LHC. These could significantly enhance the decay rate, up to existing experimental limits, but could also suppress it, as the charm-quark contribution to the K-long decay did. In the same way, the long-running search for the very rare decay of a B_s⁰ meson into a muon-antimuon pair was motivated by the search for new physics. According to the Standard Model, the expected decay rate of K_S⁰ →μμ is about thousand times smaller than that of B_s⁰→μμ, making the search particularly challenging. The analysis required a sophisticated online (real-time) selection, and made use of machine-learning tools and parallel processing with graphics processing units (GPUs). In the absence of a significant signal (see image), an upper limit on the branching fraction of 2.1 × 10^-10 is obtained, four times more stringent than the previous best limit, which was also set by LHCb.

Read more in the LHCb conference presentation, in the conference note, in the CERN Courier article and in the LHCb paper.

(2) Observation of parity violation and search for CP-violation in Λ_b⁰→pπ^-π⁺π^- decays. Charge-parity (CP) violation - a difference in behaviour between matter and antimatter - is a well-established phenomenon in the decays of K and B mesons. Recently, it has also been observed by the LHCb collaboration in the decays of D mesons. However, CP violation has yet to be established in baryonic decays. Similarly, parity violation is well established in weak interactions, but has never been observed in b-hadron decays. (In weak decays of hadrons, parity violation depends on the hadron’s constituents.)

Physicists from LHCb reported the first observation of parity violation in b-baryon decays, with a significance of over 5 standard deviations, using a sample of Λ_b⁰→pπ^-π⁺π^- decays. Searches for CP violation were also performed, giving results that are marginally compatible with the hypothesis of no-CP violation.

The left image above shows the pπ^-π⁺π^- invariant mass spectrum with an accumulation at the Λ_b⁰ mass. The other images show measured asymmetries by grouping data in different ways (different binning schemes), with horizontal black dashed lines at zero representing the hypothesis of P and CP conservation. The open blue points are inconsistent with this null hypothesis and show clear evidence of parity violation, the first observation of the phenomenon in b-baryon decays, while the red full points do not show evidence of CP violation when a statistical test is applied. A second, complementary analysis method is used in which, rather than grouping the data into bins, a single test is used to measure how well the underlying distributions of data points overlap. The result obtained from this method constitutes an observation of parity violation with over 5 standard deviations significance.

Read more in the LHCb conference presentation, in the LHCb CERN seminar and in the paper. This result is an update to a previous paper, which used about four times less signal.

(3) Search for the doubly charmed baryon Ξ_cc⁺. The LHCb collaboration discovered the exceptionally charming particle Ξ_cc⁺⁺ in 2017. It is a baryon containing two charm quarks and one up quark (ccu), resulting in an overall doubly positive charge. It is the doubly charmed counterpart of the well-known lower-mass Ξ⁰ baryon, which is composed of two strange quarks and an up quark (ssu). The Ξ_cc⁺⁺ was then rediscovered in different decay mode and its lifetime has been measured.

Other, similar combinations of quarks are also possible. If the u quark of a Ξ_cc⁺⁺ baryon is replaced by a d quark, one obtains a singly charged Ξ_cc⁺ baryon (ccd), which is expected to have a similar mass but a much shorter lifetime according to a number of theoretical calculations. LHCb physicists reported the results of a search for this particle in the decay chain Ξ_cc⁺→Λ_c⁺K^-π⁺, Λ_c⁺→pK^-π⁺, using the full Run1 and Run2 data sample.

The left image above shows the pK^-π⁺ invariant mass spectrum in which events accumulate around the Λ_c⁺ mass. Milions of Λ_c⁺ baryons are selected with high purity. The right image shows the Λ_c⁺K^-π⁺ invariant mass spectrum. A search is carried out for a possible "bump" that would indicate evidence for the Ξ_cc⁺ baryon in the “RS” (right-sign) distribution; the “WS” (wrong-sign) spectrum is a control channel where no structure is expected to show up. No significant signal is observed and, since LHCb's ability to reconstruct these particles depends strongly on how much the Ξ_cc⁺ baryon flies before decaying, a suite of limits are reported under different Ξ_cc⁺ lifetime hypotheses. The dashed blue line indicates the mass of the Ξ_cc⁺⁺ baryon discovered by LHCb, and the dotted red line indicates the mass of the Ξ_cc⁺ baryon reported by SELEX experiment at Fermilab.

Read more in the LHCb conference presentation and in the LHCb paper.

13 July 2019: Observation of two new beauty baryon particles.

Today at the EPS-HEP conference, the LHCb collaboration announced the discovery of two new beauty baryon particles. These particles are interpreted as excited states of the Λ_b⁰ baryon. The new structures are observed in the Λ_b⁰π⁺π^- spectrum using the full LHCb Run 1 and Run 2 data set, corresponding to an integrated luminosity of 9fb^-1.

The lightest baryon, the proton, which is the nucleus of the hydrogen atom, is composed of three light quarks uud while its neutral partner the neutron is composed of udd quarks. By replacing one of the d quarks by a heavier strange quark s we obtain a Λ⁰ particle composed of uds quarks. Furthermore by replacing in the Λ⁰ baryon the s quark by a charm quark c or a beauty quark b we obtain a Λ_c⁺ or a Λ_b⁰ baryon particle. The three quarks udb forming the Λ_b⁰ are in their lowest quantum mechanical state. Like electrons in atoms quarks can form excited states with different values of angular momentum and quark spin orientation. Earlier, in 2012, LHCb announced the observation of the two excited states of Λ_b⁰ baryon, Λ_b(5912)⁰ and Λ_b(5920)⁰, discovered in the Λ_b⁰π⁺π^- invariant mass spectrum.

In the analysis reported today Λ_b⁰ baryons are formed from Λ_c⁺π^- combinations, where the Λ_c⁺ baryon is reconstructed using its pK^-π⁺ decay. Λ_b⁰→J/ψK^- decays, with J/ψ→μ⁺μ^-, are also used as a cross-check. The left image below shows the Λ_b⁰π⁺π^- invariant mass spectrum with Λ_b⁰ baryons reconstructed in these two cases. A clear peaking structure is observed at a mass of around 6150 MeV in both cases.

Since the mass of the new structure is above the thresholds (minimum invariant mass) of both the Σ_b^±π^∓ and Σ_b^*±π^∓ combinations, it can potentially decay via these intermediate resonances. To investigate this the Λ_b⁰π⁺π^- mass spectrum is studied in the Λ_b⁰π^± mass regions populated by the Σ_b^(*)± resonances. The data with Λ_b⁰→Λ_c⁺π^- are split into three samples: events with a Λ_b⁰π⁺ mass around the Σ_b^± mass region, around Σ_b^∗± mass region and the remaining nonresonant (NR) ones. The Λ_b⁰π⁺π^- mass spectra in these three samples are shown in the central image above. The spectra in the Σ_b and Σ_b^* regions look different and suggest the presence of two narrow peaks at different masses, denoted in the LHCb result as Λ_b(6146)⁰ and Λ_b(6152)⁰. The right image above shows the corresponding Λ_b⁰π⁺ and Λ_b⁰π^- invariant mass spectra in the decays of new particles. As well as nonresonant component, clear contributions from intermediate resonances are visible: Λ_b(6152)⁰→Σ_b^±π^∓, Λ_b(6152)⁰→Σ_b^*±π^∓ and Λ_b(6146)⁰→Σ_b^*±π^∓. No significant evidence is found for Λ_b(6146)⁰→Σ_b^±π^∓.

The masses of the two states measured in this analysis are consistent with theoretical predictions for the 1D doublet of Λ_b⁰ states. While the newly discovered states are denoted as Λ_b⁰, their interpretation as other excited beauty baryons, such as neutral Σ_b⁰ states, can not be excluded.

Recently LHCb physicists reported the first observation of the two new mass peaks in the Λ_b⁰π⁺ and Λ_b⁰π^- invariant mass spectrum consistent with the excited Σ_b(6097)^± charged baryon.

Read more in the LHCb EPS-HEP presentation and in the LHCb paper.

28 June 2019: Updated measurement of the CP-violating phase φ_s. φ_s = -0.041±0.025 rad (LHCb)
φ_s = -0.055±0.021 rad (World)

The LHCb collaboration has submitted for publication an updated measurement of the CP-violating phase φ_s in B_s⁰ meson decays. The precision measurement of φ_s is one of the most important goals of the LHCb experiment.

In the wonderful world of quantum mechanics a B_s⁰ meson can decay directly or oscillate into B_s⁰ meson and then decay. In analogy to the two-slit quantum mechanics experiment, these two modes of decay can interfere. Interference like this is one of the key ingredients for CP violation to occur. CP violation means that particles and their antiparticles have different properties and behave in different ways; it's required to explain why the universe we live in consists mainly of matter rather than antimatter. In this case, the CP violation would manifest itself as a nonzero value of the phase φ_s.

Within the Standard Model, the value of φ_s can be calculated precisely from other measurements. The predicted value of φ_s is small, about -0.037 rad, and New Physics effects could therefore change its value significantly.

In order to obtain this new result, LHCb physicists measured the decay-time-dependent CP asymmetry in B_s⁰→J/ψK⁺K^- decays using proton-proton collision data collected with the LHCb detector at a centre-of-mass energy of 13TeV in 2015 and 2016 of the LHC Run 2, corresponding to an integrated luminosity of 1.9fb^-1. The CP-violating phase φ_s was measured using a sample of approximately 117 000 decays with a K⁺K^- invariant mass in the vicinity of the φ(1020) meson. The images below show (a) the J/ψK⁺K^- invariant mass distribution peaking at the B_s⁰ meson mass, and (b) that of the K⁺K^- combination accumulated around the φ meson mass.

The reported value is φ_s = -0.083±0.041±0.006 rad, consistent with expectations based on the Standard Model and with a previous LHCb analysis of this decay using data recorded at centre-of-mass energies 7 and 8 TeV in the LHC Run 1. The combination of the Run 1 and Run 2 results gives φ_s = -0.080±0.032 rad. This result is then combined with the recently published measurements of φ_s using B_s⁰→J/ψπ⁺π^- decays obtained with the same dataset, and with previous independent LHCb results. The combined value φ_s = -0.041±0.025 rad is consistent with expectations based on the Standard Model, and is the most precise experimental determination.

The phase φ_s is not the only interesting parameter that can be measured with B_s⁰-B_s⁰ oscillations. Following the rules of quantum mechanics, each of the particles can be expressed as a different combination of two quantum states that have slightly different masses and decay widths (and thus slightly different lifetimes). Their mass difference, Δm_s, determinates how fast B_s⁰ and B_s⁰ oscillate into each other, see 15 march 2011 news. The difference of their decay widths, ΔΓ_s, was measured in this new analysis together with the value of φ_s and other parameters such as the difference between the B_s⁰ and B⁰ lifetimes.

The left image above shows the various LHCb measurements of φ_s used for the combined result, and the combined contour (in blue) in the φ_s-ΔΓ_s plane. The B_s⁰→J/ψK⁺K^- (magenta) and B_s⁰→J/ψπ⁺π^- (red) contours are both marked 4.9 fb^-1 and represent combinations of Run 1 and Run 2 data. The Standard Model expectations obtained by the CKMfitter group is indicated by the thin black rectangle (and a similar result is also obtained by the UTFit collaboration). The right image above shows the comparison of the LHCb combined result (in green) with results from other experiments, including the recent ATLAS measurement (in blue), and the world combination by the HFLAV group (in white). The world combination gives φ_s = -0.055±0.021 rad.

Read more in the LHCb paper and LHCb CERN seminar.

26 March 2019: Observation of new pentaquarks. m(P_c⁺(4312)) = 4311.9±0.7+6.8/-0.6 MeV, Γ = 9.8±2.7+3.7/-4.5 MeV
m(P_c⁺(4440)) = 4440.3±1.3+4.1/-4.7 MeV, Γ = 20.6±4.9+8.7/-10.1 MeV
m(P_c⁺(4457)) = 4457.3±0.6+4.1/-1.7 MeV, Γ = 6.4±2.0+5.7/-1.9 MeV

Today at the Rencontres de Moriond QCD conference, the LHCb collaboration announced the discovery of a new narrow pentaquark particle, P_c(4312)⁺, decaying to a J//ψ and a proton, with a statistical significance of 7.3 standard deviations. In addition, the P_c(4450)⁺ pentaquark structure previously reported by LHCb is also confirmed, but a more complex structure consisting of two narrow overlapping peaks, P_c(4440)⁺ and P_c(4457)⁺, is now emerging, with the two-peak structure having a statistical significance of 5.4 standard deviations compared to a single-peak hypothesis.

In the conventional quark model, strongly interacting particles known as the hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons. In their fundamental 1964 papers [1] and [2], in which they proposed the quark model, Murray Gell-Mann and George Zweig mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration. It took 50 years, however, for physicists to obtain unambiguous experimental evidence of the existence of these exotic hadrons. In April 2014 the LHCb collaboration published measurements that demonstrated that the Z(4430)⁺ particle, first observed by the Belle collaboration, is composed of four quarks (ccdu). Then, a major turning point in exotic baryon spectroscopy was achieved at the Large Hadron Collider in July 2015 when, from an analysis of Run 1 data, the LHCb collaboration reported significant pentaquark structures in the J/ψp mass distribution in Λ_b⁰→ J/ψpK^- decays.

Various interpretations of these structures have been proposed, including tightly bound pentaquark states and loosely bound molecular baryon-meson state. These two possibilities are illustrated in the figure to the left. The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The leftmost image illustrates how the quarks could be tightly bound; the image to the right shows a loosely bound meson-baryon molecule, in which a meson and a baryon are connected by a residual strong force similar to the one that binds protons and neutrons together within nuclei.

The analysis presented today used the combined data set collected by the LHCb collaboration in Run 1 (with pp collision energies of 7 and 8TeV, and corresponding to a total integrated luminosity of 3 fb^-1) plus Run 2 (6 fb^-1 at 13TeV). From this sample, 2.5x10⁵ Λ_b⁰→ J/ψpK^- decays were selected, nine times more than in the previous Run 1 analysis. The combined data set was analysed in the same way as in the earlier 2015 paper and the parameters of the previously reported P_c(4450)⁺ and P_c(4380)⁺ structures were found to be consistent with the original results. However, analysis of the much larger data sample reveals additional peaking structures in the J/ψp invariant mass spectrum which were not visible in the data sample used before. A narrow peak is observed near 4312MeV with a width comparable to the mass resolution. The structure at 4450MeV is now resolved into two narrow peaks, at 4440 and 4457MeV. The images below show the contribution of these pentaquark states to the J/ψp invariant mass spectra.

The minimal quark content of these states is duucc: four quarks and one antiquark. Since all three states are narrow and lie just below the Σ_c⁺D⁰ and Σ_c⁺D^*0 thresholds (meaning that their mass is slightly smaller than the sum of the masses of a Σ_c⁺ and a D⁰ or a D^*0) by amounts that correspond to plausible hadron-hadron binding energies, they provide a possible experimental evidence for the existence of bound states of a baryon and a meson, as seen in the image above. If this interpretation is correct, the decay channels open to the states would be restricted. Being just below threshold, such states would not decay by "falling apart" into a Σ_c⁺ baryon and a D⁰ or a D^*0 meson, but could decay instead to a J/ψ meson and a proton. In the baryon-meson configuration shown in the image, it is not easy for the c and c quarks to come close enough together to form a cc bound state (i.e. a J/ψ meson). Therefore, such a baryon-meson configuration is expected to be relatively stable and would be observed as a narrow peak, following the basic rules of quantum mechanics. A description of these states as tightly bound clusters of five quarks is also plausible. A full understanding of the internal structure of the observed states will require more experimental and theoretical study.

Read more in the Moriond presentation, in the LHCb paper, in the CERN news and in the LHCb CERN seminar.

22 March 2019: Update of lepton universality test measurement R_K.

Today at the Rencontres de Moriond EW conference, the LHCb Collaboration presented an updated measurement of the ratio R_K, an important test of a principle of the Standard Model of particle physics known as "lepton universality", which states that the Standard Model treats the three charged leptons (electrons, muons and taus) identically, except for differences due to their different masses.

The ratio R_K describes how often a B⁺ meson decays to a charged kaon and either a muon and anti-muon pair (K⁺μ⁺μ^-) or an electron and anti-electron pair (K⁺e⁺e^-). These decays are extremely rare, occurring at a rate of only one in two million B⁺ meson decays. The decays involve the transformation of a beauty quark into a strange quark (b→s), a process that is highly suppressed in the Standard Model and can be affected by the existence of new particles, which could have masses too high to be produced directly at the Large Hadron Collider.

LHCb has studied a number of other such ratios comparing decays with different leptons in beauty particle decays (see R_K, R_K^*0, R(D^*) and R(J/ψ)). These results revealed hints of deviations from lepton universality, none of which was significant enough to constitute evidence of new physics on their own. However, according to theorists who study possible extensions of the Standard Model, taken together these deviations suggest an interesting and coherent pattern. All previous LHCb results used only the Run 1 data sample. During Run 2 (2015-2018), LHCb collected a much larger data sample containing approximately four times larger number of beauty particle decays and these data are now being analysed intensively. Measurements like R_K use the technique of blind analysis, in which the physicists analysing the data do not know the result until the analysis method is finalized and frozen, following an extended review within the collaboration. The measurement presented today is the first lepton universality test performed using part of the Run 2 data set (2015-2016) together with the full Run 1 data sample, representing in total an integrated luminosity of 5fb^-1.

To minimise the influence of detector and other experimental effects, LHCb physicists used a "double ratio" method: what they measure is R_K divided by another ratio, r_J/ψ, the true value of which is known to be very close to 1 but which has similar sensitivity to detector effects to R_K. This gives an extra layer of protection: the scientists study and correct for all known experimental effects, but if any unknown effects slip through they will cancel in the double ratio between R_K and r_J/ψ. In more detail, R_K is defined as the ratio of probabilities that a B⁺ meson decays to K⁺μ⁺μ^- or K⁺e⁺e^- (within a particular invariant mass range; see below), and r_J/ψ is defined as the ratio of probabilities that a B⁺ meson decays to J/ψK⁺ with J/ψ →μ⁺μ^- vs J/ψK⁺ with J/ψ →e⁺e^-. The double ratio method greatly reduces systematic uncertainties related to the different experimental treatment of muons and electrons, which largely cancel in the double ratio. In particular, the use of the double ratio method means that the detection efficiency of the decay B⁺→K⁺e⁺e^- only needs to be known relative to that of the B⁺→J/ψ(→e⁺e^-)K⁺ decay, rather than the B⁺→K⁺μ⁺μ^-. Moreover, J/ψ meson decays into μ⁺μ^- and e⁺e^- pairs are known to respect lepton universality at the 0.4% level. This means that the measurement of the single ratio, r_J/ψ, also constitutes an excellent cross-check, since it does not benefit from the double ratio's cancellation of systematic effects and so is a sensitive and stringent test of the methods used to determine the efficiencies. The value of r_J/ψ is found to be 1.014±0.035, consistent with 1. The figures show the measured invariant mass distributions of B⁺ candidates for the four decay modes used in the double-ratio measurement, each with a clear peak around the B⁺ meson mass.

The analysis is performed in the range 1.1q²6.0 GeV², where q² is the invariant mass of the μ⁺μ^- and e⁺e^- pair, and benefits from an improved reconstruction compared to the previous LHCb R_K measurement. The value of R_K is measured to be 0.846^+0.060_-0.054^+0.016_-0.014 , where the first uncertainty is statistical and the second systematic, and is shown at the image as a black point with error bars. This is the most precise measurement to date and is consistent with the SM expectation at the level of 2.5σ (2.5 standard deviations), a value very similar to the 2.6σ obtained in the previous measurement, and show as a grey point. The new measured value of R_K supersedes the previous one, is closer to the Standard Model prediction, and has a smaller uncertainty. The result of BaBar Collaboration at low q² is shown by the green left point and favors also a value below one. The blue point shows that the result of the R_K measurement by the Belle Collaboration is consistent with one in the whole q² range up to 22 GeV². Further reduction in the uncertainty on R_K can be expected when the data collected by LHCb in 2017 and 2018, which have a statistical power approximately equal to that of the entire 2011-2016 data set used here, are included in a future analysis. In the longer term, there are good prospects for even higher-precision measurements as much larger samples will be collected with the upgraded LHCb detector.

Read more in the LHCb Moriond presentation, in the LHCb paper and in the LHCb CERN seminar.

21 March 2019: Discovery of CP violation in charm particle decays. An important milestone in the history of particle physics. [ ΔA_CP = (-0.154±0.029)% ]

The LHCb collaboration has just presented at the Rencontres de Moriond EW and in a special CERN Seminar the first observation of CP violation in charm particle decays. Quarks can be split into two sectors: those with the same electrical charge as the up quark (up-type quarks, charge +2/3), and those with the same as the down quark (down-type quarks, charge -1/3). Differences in the properties of matter and antimatter, arising from the so-called phenomenon of CP violation, had been observed in the past using the decays of K and B mesons, i.e. of particles that contain strange or beauty quarks, which are both down-type quarks. By contrast, despite decades of experimental searches, CP violation in the decays of charmed particles, i.e. containing the charm quark, which is an up-type quark, escaped detection so far. The result announced today constitutes the first observation of CP violation in decays of a charmed particle.

CP violation is one of the key ingredients required to explain why today's universe is only composed of matter particles, with essentially no residual presence of antimatter. The phenomenon was first observed in 1964 in the decay of neutral K mesons, and the two physicists who made the discovery, James Cronin and Val Fitch, were awarded the Nobel Prize in physics in 1980. Such a discovery came as a great surprise at the time, as it was firmly believed by the community of particle physicists that the CP symmetry could not be violated. In the early 1970s, building on the foundations laid by Nicola Cabibbo and others some years before, Makoto Kobayashi and Toshihide Maskawa realised that CP violation could be included naturally in the theoretical framework that we know today as the Standard Model of particle physics provided that at least six different quarks existed in nature. Their fundamental idea was confirmed eventually three decades later by the discovery of CP violation in beauty particle decays by the BaBar and Belle collaborations, leading to the award of the 2008 Nobel Prize in physics to Kobayashi and Maskawa. In the Standard Model, the existence and overall size of CP violation are determined by a single parameter, though the way it manifests in a particular decay is influenced by several others. The values of these fundamental parameters can be determined experimentally by measuring many different CP-violating processes. The combined set of these measurements, many of which have been performed by LHCb, agree very well with the Standard Model predictions for all CP-violating effects known so far in particle physics. The Standard Model also predicts a tiny amount of CP violation in charm particle decays, at a level that is difficult to calculate exactly but could be up to 10^-3 - 10^-4 in decay modes of interest.

LHCb physicists studied the differences in decay rates of neutral D⁰ mesons, composed of a charm quark (c) bound by strong interactions with an up antiquark (u), and D⁰ mesons, composed of a charm antiquark (c) bound with an up quark (u). The goal of the analysis is to measure the difference between the decay rates of D⁰ and D⁰ mesons decaying into K⁺K^– pairs or into π⁺π^- pairs. In practice, the measurement consists in counting the numbers of D⁰ and D⁰ mesons decaying into K⁺K^- or π⁺π^- pairs which are present in the data sample recorded by LHCb in 2011-2018. The LHCb experiment collected an unprecedented amount of such charm decays, allowing physicists from the LHCb collaboration to pinpoint the tiny size of CP violation in the charm-quark sector. However, these decays are identical for D⁰ and D⁰ mesons: how can you tell if the decaying meson is D⁰ or D⁰ to understand whether there are more D⁰ or D⁰ mesons decaying to K⁺K^- (or π⁺π^-) pairs?

To answer this question LHCb physicists exploited two different classes of decays:

a) D⁰ and D⁰ mesons are produced from D^*+(-) meson decays, D^*+ → π⁺D⁰ and D^*- → π^-D⁰. The presence of a π⁺ at this point in the decay chain indicates the presence of a D⁰ meson, whereas a π^- accompanies a D⁰ meson.

b) D⁰ and D⁰ mesons are produced from so-called semileptonic beauty decays, as for example B⁺ → μ⁺νD⁰ or B^- → μ^-νD⁰. In this way, the presence of a μ⁺ identifies a D⁰ meson, whereas a μ^- indicates a D⁰.

The images show the so-called invariant-mass distributions used to count the number of decays that are present in the data sample. The area of each blue, bell-shaped (Gaussian) peak is proportional to the number of decays of that type recorded by the experiment. The final result, which uses essentially the full data sample collected by LHCb so far, is given by the quantity ΔA_CP=(-0.154±0.029)%, whose difference from zero quantifies the amount of CP violation observed. In particular, this turns out to differ from zero by 5.3σ (5.3 standard deviations), thus surpassing the threshold of 5σ adopted by particle physicists to assert without reservation that a discovery is made. This represents the first observation of CP violation in charm particle decays, opening up a new field in particle physics: the study of CP-violating effects in the sector of up-type quarks, and searches for new physics effects using charm CP asymmetry measurements.

Read more in the Moriond EW presentation, in the CERN seminar presentation, in the LHCb paper, in the CERN Press Release in English end French, in The Conversation article.

15 March 2019: Observation of an excited charmed beauty particle. Observation of an excited B_c⁺ state.

This week the LHCb collaboration announced the observation of an excited B_c⁺ state, confirming measurements by the ATLAS and CMS collaborations. The results were presented at the conference “Rencontres de Physique de la Vallée d'Aoste” held at La Thuile, Italy.

The B_c⁺ particle is a very interesting meson composed of two different heavy quarks: an anti-beauty quark (electric charge +1/3) and a charm quark (+2/3) bound together by the strong nuclear force. Just like ordinary atoms, c and b quarks can be arranged in various quantum states with different angular momenta and spin configurations, giving rise to a spectrum of particles with different masses like charmonium and bottomonium systems. The charmonium, cc, and bottomonium, bb, quark bound systems were intensively studied after the discovery of the J/ψ meson on 11 November 1974. (Indeed, LHCb continues to contribute to these studies; see a recent contribution in the news below). The study of the B_c⁺ system, cb, is much more difficult. B_c⁺ mesons are produced much more rarely, since both a cc and a bb quark pair need to be formed close together. But at the LHC, the large energy and intensity of pp collisions allow sufficient number of B_c⁺ mesons to be produced, enabling scientists to study the properties of the cb quark bound system.

B_c⁺ mesons are unstable, but they can be reconstructed by looking for their decay products. In the first step of the analysis, LHCb physicists looked for pairings of a J/ψ and a π⁺ meson, one possible B_c⁺ decay mode. The image shows the corresponding invariant mass spectrum. The points with error bars are the data, and the red cross-hatched area shows a clear contribution from the decays of the B_c⁺ ground state: the lowest-mass state of the cb system. (The ground state is sometimes just called the B_c⁺.)

In the second step of analysis, the physicists looked for signs of strong decays of excited B_c⁺ mesons to the ground state. To do this, they looked for combinations of the B_c⁺ ground-state meson plus a pair of opposite sign pions, π⁺π^-. The left image below shows the corresponding invariant mass spectrum, with the black points showing the mass of B_c⁺π⁺π^- combinations: two peaking structures are visible. The same image also includes a red histogram; this represents combinations of a B_c⁺ with two same-sign points, e.g. B_c⁺π⁺π⁺. Because these same-sign combinations have the wrong electric charge, they cannot contain real excited B_c⁺ states and are useful as a measure of the background. The right image below is made from the same data sample but plotted in a different way: the x-axis variable, ΔM, is obtained by subtracting the B_c⁺ meson mass from the B_c⁺π⁺π^- invariant mass. The two coloured peaks show the contributions of the two excited states announced at the conference today: the left one (red hatched) is observed with a significance above 5σ while the right one (red cross-hatched) has a local significance above 3σ. The numerical results of the measurements can be found in the LHCb presentation.

What are these structures? The four relevant quantum states of the cb system are show in the image to the left. Like many systems in atomic, nuclear, and particle physics, angular momentum plays a key role. The energy of the system is determined by three quantities: the spin arrangement of the c and b quarks, the angular momentum between the quarks, and the radial excitation (size) of the system. Each of the four states shown is marked "S", meaning that there is zero orbital angular momentum between the quarks. There are two main levels: a ground state 1S and an excited state 2S. Within each of those two levels, the quarks can be arranged so that their spins are opposed (↑↓) or aligned (↑↑). Since each quark has spin ½, these correspond to total spins of 0 or 1; like when putting bar magnets side by side, the arrangement with opposed spins is more stable and corresponds to a lower-mass state. The two peaks seen by LHCb are due to the two decays of a 2S excited state to a 1S ground state (2S₀→1S₀ and 2S₁→1S₁) accompanied by a pair of pions (π⁺π^-), shown as vertical black arrows in the figure. For the 2S₁ decay chain, there is an extra photon produced (marked γ, from the transition (1S₁→1S₀ at the bottom of the figure) but it is not reconstructed.

There is a correspondance between the four cb states shown in the image and the charmonium states η_c(1S), η_c(2S), J/ψ and ψ(2S) shown in the schema in the recent LHCb news below. The cb state 1S₁, corresponding to the J/ψ meson in this analogy, has not yet been observed directly.

The B_c⁺ meson was first observed by the CDF collaboration at the Tevatron collider in 1998. The ATLAS collaboration first reported a single peak in the B_c⁺π⁺π^- invariant mass distribution in 2014, but due to experimental limitations on the mass resolution and signal yield the ATLAS analysis was unable to resolve the two-peak structure. One month ago, in February 2019, the CMS collaboration released a paper in which the two peak structure is resolved. The LHCb and CMS results for the properties of the peaks are consistent.

Properties of the B_c meson family can reveal information on heavy quark dynamics and improve the understanding of the strong interaction in the intermediate energy region between charmonium and bottomonium. The two –onium systems have been studied for 40 years. The precise study of B_c meson family, the cb system, starts now. The LHCb experiment at the LHC is very well designed to play a leading role in this investigation.

Read more in the LHCb presentation and in the LHCb paper.

26 February 2019: Observation of a new charmonium state … and study of near-threshold D⁰D⁰ spectroscopy. [m_X(3842) = 3842.72±0.16±0.12 MeV/c²; Γ_X(3842) = 2.79±0.51±0.35 MeV ]

This week the LHCb Collaboration announced the discovery of a new particle made of a charm and anti-charm quark. The results, which were presented at the International Workshop "e⁺e^- Collisions From Phi to Psi 2019" held in Novosibirsk, are the first to make use of all the available data recorded by the LHCb experiment from 2011 to 2018, and also include precise measurements of the properties of two other so-called "charmonium" states.

The charmonium states are bound systems of a charm quark, c, and an anti-charm quark, c, held together by the strong nuclear force. Just like ordinary atoms, e.g. hydrogen, c and c quarks can be arranged in various quantum states with different angular momenta and spin configurations, giving rise to a spectrum of particles with different masses, all composed of the same fundamental quarks. In recent years there has been a resurgence of interest in charmonium spectroscopy following the discovery of states that do not fit into the conventional charmonium spectrum. It is sometimes difficult to conclude if a new discovered state is a previously unobserved charmonium state or an exotic particle composed e.g. of four quarks, such as a tetraquarks. Knowledge of the spectrum of conventional states is important to help identify exotic states: if all predicted conventional states are accounted for, we can be more confident that the remaining ones are exotic.

Charmonium states with mass smaller than twice the mass of a charm meson D cannot decay into a pair of charmed particles that each contain a charm c or an anticharm c quark. Instead, the states with higher masses can, and decay typically into charm meson pairs. LHCb physicists studied the decays of charmonium in D⁰D⁰ and D⁺D^- meson pairs. The image above shows the corresponding invariant mass spectra. Several peaking structures can be clearly seen and are listed in the figure. The images below show fits to different regions of the mass spectra with models used to describe these structures as charmonium states emerging above the background. The narrow structure marked X(3842) in the image above represents the contribution of a new narrow charmonium state, observed for the first time by LHCb, with overwhelming statistical significance in both decay modes. The left image below shows a fit giving the mass of this state to be 3842.72±0.16±0.12 MeV/c² and the natural width 2.79±0.51±0.35 MeV, where the first error is statistical and the second systematic. The observed mass and narrow natural width suggest the interpretation of the new state as the previously unobserved ψ₃(1D) charmonium state. This represents the first spin-3 charmonium state observed. It is interesting to note that LHCb discovered in July 2014 another spin-3 state in the D_s meson system.

The other peaking structures visible in the images above represent prompt hadroproduction, seen for the first time, of the previously observed ψ(3770) and χ_c2(3930) states. The most precise measurements ever of the resonance parameters of these states were made. The low mass structure visible in the D⁰D⁰ invariant mass spectrum comes from the χ_c1(3872)→D^∗0D⁰ decays followed by the D^∗0→D⁰π⁰ or D^∗0→D⁰γ decays, where the γ and the π⁰ are not detected.

The left image shows LHCb contribution to the knowledge of the charmonium spectrum. The newly discovered X(3842) state is marked in red as ψ₃(1D). The two states, which new parameter measurements were announced at the conference, are marked in green. Other states, in which the most precise measurements comes from LHCb, are shown with blue lines. In particular, LHCb announced in September 2017 a very innovative measurement of the masses of the χ_c1 and χ_c2 charmonium mesons, performed for the first time by utilising the newly discovered decay χ_c1,2→μ⁺μ^-J/ψ. Two not-yet discovered states h_c(2P) and η_c2(1D) are shown with dashed lines. The DD threshold is indicated with dotted line.

Read more in the LHCb presentation, in the CERN news in English and in French and also in the LHCb paper.

11 December 2018: Celebrating a masterpiece of particle physics and Industry Awards.

The LHC Run 2 ended on December 3, see the corresponding news. The current experiment will be largely dismantled and an almost completely new detector constructed during the two-year-long shutdown: a new LHCb will be born in 2021. The major fraction of LHCb's sub-detectors will be replaced or upgraded during this shutdown. Last week about 500 members of the collaboration celebrated the end of the first stage of the experiment with the detector in its original form and the beginning of construction of a new detector. There were memorable sessions reminding members of the collaboration how the ideas of the first dedicated beauty experiment at the hadron collider were born and developed as well as on the history of different sub-detector construction.

They were followed by a fantastic party with amazing number of artists giving terrific musical performances, accompanied by video1, video2 and a slide show presenting images of the old detector and preparations for the installation of a new one. The image shows a musical ensemble playing “A Requiem for LHCb” during the celebration.

The construction of the new upgraded detector will profit from the most recent technological developments. LHCb physicists and engineers have been assisted in their efforts by excellent companies that have collaborated closely with LHCb and their contributions have been crucial in ensuring the successful upgrade of the experiment. The LHCb collaboration selected three outstanding companies, Kuraray, Europractice and Advacam, for LHCb Industry awards and presented these at a special ceremony which took place during celebration.

The images above show the celebration speech given by the Collaboration Board chairperson Valerie Gibson and the spokesperson Giovanni Passaleva and the presentation of the Industry Awards to the winners.

The Kuraray company delivered 12 000 km of scintillating plastic fibre for the LHCb Scintillating Fibre Tracker (SciFi) detector. The fibre is in principle a product available in Kuraray's catalog, however, LHCb’s technical requirements in terms of attenuation length, light yield and geometrical quality significantly exceeded their standard specifications. The company had to improve production and environmental parameters and invest in measurement and quality control equipment. The stable quality of the product over the full delivery period was excellent, the photo and video shows the successful quality tests being performed at CERN.

Europractice helped to design and produced the CLARO ASIC, a 8-channel amplifier/discriminator for single photon counting with multi-anode photomultiplier tubes. The ASIC, an Application-Specific Integrated Circuit, is an integrated circuit customized for a particular use, rather than intended for general-purpose use. The CLARO is designed to sustain high rate, up to 40 Mhits/s per pixel, with a low power consumption, and it is one of the main building blocks of the LHCb RICH upgrade. Europractice provided access to the software tools needed for ASIC design, and to the silicon foundry for prototyping and production. Nearly 100k chips were produced in total according to the specifications in terms of speed, power consumption and radiation tolerance, and these fulfill the requirements of the LHCb RICH upgrade. The images shows the CLARO ASIC.

Advacam developed a novel thinned ASIC bump bonding technique for the Upgrade of the Vertex Locator of the LHCb detector. The company had a very collaborative approach to the RD and prompt, large scale delivery of devices in full conformity with tender. LHCb very much appreciated the close collaboration of the company, with weekly meetings and feedback on all aspects of project. The excellent quality control achieved was particularly impressive. The image shows the interconnection bumps of the VeloPix ASIC.

A very busy period starts now. Members of the collaboration will continue to analyse Run1 and Run2 data and, at the same time, finalise construction and then install the new detector. The road towards new discoveries continues.

3 December 2018: End of the first phase and beginning of a new LHCb. End of 2018 data taking period marking the end of Run 2.

At the end of every year, the CERN accelerator complex ends its operation for the usual winter shutdown, in which particle accelerators and experiments perform necessary maintence. But today, when the LHC stopped at 4:38 am, marks the beginning of something very different: the end of the LHC's Run 2 operation period. After a two-year-long break known as Long Shutdown 2 (LS2), the collider will restart again in 2021 for the Run 3 operation period. For LHCb, today is the end of data taking with detector in its original form. The current experiment will be largely dismantled and an almost completely new detector constructed during the LS2, and a new LHCb will be born in 2021.

The 2018 data taking period was divided into two parts, and was extremely successful. Proton-proton (pp) collisions started on April 28^th and were followed by the lead-ion-lead-ion (PbPb) collisions starting on November 9^th. The image shows comparison of integrated luminosity recorded by LHCb during different pp data taking periods. This year LHCb recorded 2.19 fb^-1, the best performance ever achieved and slightly higher than the value obtained in 2012, the last year of Run 1. The total luminosity collected in Run 2 is nearly 6 fb^-1, twice the Run 1 sample of 3 fb^-1. Moreover, since the cross-section for b- and b-quark production at 13 TeV proton-proton collisions is about twice that of Run 1 (7 and 8 TeV), the number of beauty particles available for physics analysis is four times higher in the Run 2 data than in Run 1. Excellent perspectives for more and more precise physics results and for the exploration of so-far-inaccessible rare decays are therefore opening for LHCb.

LHCb, the world's first dedicated b-physics experiment at a hadron collider, is not only producing world-leading results in heavy flavour physics, but it is obtaining important results also in other fields thanks to its excellent detector performances and unique large-rapidity acceptance that make it a general purpose detector in the forward region. LHCb collected the first pPb collision data in 2013 and the first PbPb data in 2015. The 2018 PbPb data taking period was exceptionally successful. The number of collisions per unit time (instantaneous luminosity) was up to 50 times higher than was seen in 2015, thanks to the number of colliding bunches (468) beeing 20 times higher and the focalization of colliding Pb beams beeing twice better, while the overall number of collected events (integrated luminosity) was 20 times higher. The image shows a clearly visible J/ψ peak reconstructed during data taking from the PbPb collisions and from the Pb beam interactions with the injected Ne gas target. LHCb physicists are now eagerly awaiting the offline data processing to start exploring this large PbPb collision data sample.

In pp data taking, bunches of protons can cross every 25 ns at the LHC, corresponding to a frequency of 40 MHz (with about 30 MHz of events with collisions). During Run 1 and 2 the event rate was filtered down to 1 MHz with the help of fast electronics, using comparatively simple algorithms to select the most interesting events. Those events were then processed in a dedicated computer farm, located underground close to the detector; this allowed additional, more sophisticated selection criteria to be applied using software (see the 14 October 2015 news for more). For Run 3 and beyond, this will change radically: the fast electronics will be removed and the whole detector will be read out at the full rate of 40 MHz. This will allow the whole selection to be done in software, meaning that it can be much more precise and flexible. Better yet, from Run 3 the pp collision rate at LHCb will be increased by a factor of 5 (for experts: the luminosity will rise to 2x10³³cm^-2s^-1). With the higher luminosity and a greatly improved ability to pick out the most interesting events, LHCb can look forward to much larger signal yields.

A major fraction of LHCb's sub-detectors will be replaced or upgraded during LS2 in order to cope with these much more demanding data-taking conditions, using the most recent technological developments to design the new detectors. The VErtex LOcator (VELO) will be replaced by a new silicon pixel detector that will come as close as 5.1 mm to the proton beams (see more). The tracking detectors will be replaced by a new high-granularity silicon micro-strip detector, the Upstream Tracker (UT), placed upstream of the magnet, and by the three stations of the Scintillating Fibre Tracker (SiFi), which is placed downstream the magnet, and consists of 2.5m-long scintillating plastic fibre matrices read out by silicon photo-multipliers. The mirrors of the RICH1 detector will have a larger curvature radius, and the current Hybrid Photon Detectors (HPD) will be replaced by multianode photomultipliers in both RICH1 and RICH2 detectors. The scintillating pad detector (SPD), the preshower (PS) and the first muon chamber (M1) will be removed. The electronics connected directly to the detector (front-end) of all sub-detectors will be modified. And last but not least, the computing power of the LHCb software event selection system (trigger) will be significantly increased as mentioned above, and the entire readout system together with the computer farm will be moved from underground to the surface. The images above show the preparation of a test of the new VELO (left), a bundle of scintillating fibres (right) and the schematic view of the upgrated detector.

Read more in the CERN Press Release in English and French, in the CERN Couriel article and also in the CERN news in English and French.

27 September 2018: New particles and precise measurements.

The LHCb Collaboration presented new interesting results at the Large Hadron Collider Committee (LHCC) open session and at the CKM workshop in Heidelberg. Selected topics are listed below.

(1) Observation of two new particles in the Λ_b⁰π^± system. LHCb physicists observed and studied two new Σ_b particles—as well as four known ones—in the invariant mass spectrum of the two-body system Λ_b⁰π^±, consisting of a neutral Λ_b⁰ baryon and a charged π meson. These Σ_b particles manifest as peaks above the smooth background, as shown in the image; on the x-axis, Q=m(Λ_b⁰π^±)-m(Λ_b⁰)-m(π^±). The Λ_b⁰ baryons are reconstructed via their decay Λ_b⁰→ Λ_c⁺π^-, with the Λ_c⁺ baryons in turn decaying to pK^-π⁺.

In the quark model, the Λ_c⁺ and Λ_b⁰ are each composed of three quarks: udc for Λ_c⁺ and udb for Λ_b⁰. They are heavier partners of the well-known strange baryon Λ⁰, which is composed of uds quarks and whose discovery pre-dates the Standard Model. The Σ_b family of resonances have slightly different combinations of light quarks: uub for Σ_b⁺ and ddb for Σ_b^-. In each of the upper plots, there are two peaks clearly visible (blue) that are identified as the Σ_b^± and Σ_b^*± baryons, which were observed previously by the CDF collaboration. LHCb confirms the CDF results, improving the precision on those particles' properties by a factor of 5. Beyond these two particles, additional excited states are expected at higher masses. The lower images show mass spectra over a wider range and with stricter selection requirements. The peaks in the lower plots (pink) represent the first observation, with significances of 12.7σ and 12.6σ, of two new particles named the Σ_b(6097)⁺ and Σ_b(6097)^-. They are likely to be part of same family of excited states of the Σ_b baryons. Theoretically, the family of excited Σ_b states whose mass is closer to that of the new observed particles is composed of five Σ_b(1P) states, some of which may be difficult to observe experimentally. Since it's possible that the masses of different excited states may be similar, it cannot be excluded that the structures seen are superpositions of more than one state.

The search for and study of these states will cast light on the internal mechanisms governing the dynamics of the strong force that binds quarks inside hadrons. Read more in the LHCb presentation, in the LHCb paper, in the CERN update in English and French and in the CERN Courier article.

(2) Evidence for an exotic particle decaying into η_c(1S)π^-. In the quark model, strongly bound particles (hadrons) are formed from combinations of quarks (q) and antiquarks (q) that have no overall colour charge. Until relatively recently, only two classes of hadron were known experimentally: qq pairs (mesons) and sets of three quarks qqq or qqq (baryons and antibaryons). These are referred to as conventional hadrons; those which do not fit into either of the categories are called exotic hadrons. The LHCb collaboration made important contributions through the study and discovery of exotic particles, like exotic mesons (which could be tetraquarks) composed of two quarks and two antiquarks qqqq, and pentaquarks, composed of four quarks and one antiquark qqqqq. An interesting feature of the exotic particles is that they contain a heavy charm quark-antiquark pair, cc, and therefore charmonium mesons like J/ψ or ψ(2S) (sometimes denoted ψ') were observed in their decays. (For more on those, the Particle Data Group has a review of the charmonium system in pdf format.)

At the LHCC open session, the LHCb collaboration presented the first evidence for an exotic particle decaying into another charmonium (cc) meson, the η_c(1S), plus a π^-. The η_c(1S) was reconstructed through its decay to a proton (p) and an antiproton (p). Instead of looking for any combination of an η_c(1S) and a pion, the researchers studied those from the decay of a B meson: B⁰→ η_c(1S)K⁺π^-. In the absence of exotic resonances, this B⁰ decay would proceed predominantly through intermediate kaon resonances, such as K^*0→K⁺π^-, as shown in the left Feynman diagram. The right diagram shows a possible contribution proceeding through an exotic particle denoted the Z_c^-, with minimal quark content ccdu, which then decays into the two-body system η_c(1S)π^-.

The images above show the invariant mass spectra of the η_c(1S)π^- system (from the B⁰ decays discussed above). The left image shows the results of a best description of the data based on a model using only conventional hadron resonances (particles), like those included in the left Feynman diagram. A discrepancy is visible around 4.1 GeV. The right image shows what happens when contributions from an exotic resonance, the Z_c^-(4100), are allowed: a much better description of the data is obtained. The significance of this new exotic candidate is more than 3σ when including systematic uncertainties. This result was obtained with the LHC Run 1 data plus part of the Run 2 data (4.7fb-1 in all). LHCb will have recorded approximately twice this amount of integrated luminosity by the end of Run 2, so inclusion of the rest of the Run 2 data in a future update of the analysis will improve the precision and help clarify whether this intriguing hint is indeed a new, exotic particle. Read more in the LHCb presentation, in the LHCb paper and in the CERN Courier article.

(3) Measurements of the branching fractions of rare decays of charmed mesons. LHCb physicists presented measurements of the branching fractions of rare decays (for experts: doubly Cabibbo-suppressed) of charmed mesons D_s⁺ (with quark content cs)) and D⁺ (cd)). Precise measurements of these branching fractions provide important information for the understanding of the decay dynamics of these particles. Modeling these decay dynamics for charm mesons is a challenging theoretical problem: the charm quark mass is not heavy enough for robust application of theoretical methods that are used successfully in calculations of B meson decays, but not light enough for an exhaustive, brute-force approach either (which has more success for lighter K meson decays).

Measurements of the branching fractions of the rare charm meson decays D⁺→K^-K⁺K⁺, D⁺→π^-π⁺K⁺ and D_s⁺→π^-K⁺K⁺ were reported. The distribution of D_s⁺→π^-K⁺K⁺ candidates across the Dalitz plot is shown in the image, where the presence of intermediate particles (resonances) in the decays can be identified as highly populated regions or bands. Numerical values can be found in the LHCb presentation and in the LHCb paper, and are the most precise results obtained for these decays up to date.

6 July 2018: Puzzling measurement of the Ω_c⁰ charmed baryon lifetime. [ τ(Ω_c⁰) = 268±24±10±2 fs ]

Today the LHCb collaboration submitted for publication the result of a measurement of the lifetime of the Ω_c⁰ baryon, i.e. a composite particle composed of css quarks. The Ω_c⁰ baryon was identified via Ω_b^-→Ω_c⁰μ^-ν_μX decays, and then by means of the Ω_c⁰→pK^-K^-π⁺ decay.

The decay-time distribution of the Ω_c⁰ baryons reconstructed in the analysed sample was measured relative to that of D⁺ meson decays, whose corresponding distribution is shown in the left image above. The lifetime of the D⁺ meson is known to better than 1% precision. This approach allowed for significant reductions in the systematic uncertainty associated to the measurement. A similar technique was also used, for example, in the Ξ_cc⁺⁺ lifetime measurement. The right image above shows the measured Ω_c⁰ decay-time distribution marked as “Data”. The “Fit” distribution is created with the lifetime parameter adjusted to describe (fit) the data in the best possible way.

The measured lifetime value 268±24±10±2 fs is nearly four times larger than, and inconsistent with, the values measured by previous experiments, as visible in the image aside. The Ω_c⁰ baryon lifetime was measured about 15-20 years ago by the Fermilab photoproduction experiments E687 and FOCUS and the CERN WA89 experiment located at the Σ^- beamline of the SPS. The small samples of Ω_c⁰ observed at the time were produced in collisions with nuclear targets. LHCb physicists analysed about 1000 Ω_c⁰ baryon decays, which constitute a sample that is at least an order of magnitude larger than those used by previous experiments. The spectrum in the right image above labelled as “τ=69 fs” shows what LHCb physicists should have observed if the Ω_c⁰ lifetime had the world average (PDG) value. The inconsistency is clearly visible.

It is rare that a new measurement of the properties of a known particle results in such a large difference compared to previous measurements. Since lifetime measurements of hadrons containing heavy quarks are sensitive to the internal structure and dynamics within those hadrons, today’s precision measurement by LHCb will stimulate even more interest in charmed-baryon lifetime measurements as well as renewed theoretical efforts to understand the internal structure of charmed baryons. Moreover, they will help us understand how to incorporate QCD effects into the calculations used to describe the decays of baryons containing heavy (b or c) quarks.

Read more in the LHCb publication, in the LHCb presentation at the 2018 Conference on Large Hadron Collider Physics, LHCP, in Bologna, and also in the CERN Update in English and French, in the CERN Courier article as well as in the Physics Today Research Update.

6 July 2018: Re-discovery of an exceptionally charming particle. [ m(Ξ_cc⁺⁺) = 3621.55±0.23±0.30 MeV/c² ]

A year ago the LHCb collaboration announced the observation of an exceptionally charmed particle, the Ξ_cc⁺⁺ baryon. This particle contains two charm quarks and one up quark, resulting in an overall doubly positive charge. A month ago the collaboration has presented the first measurement of the lifetime of this baryon. Today LHCb physicists submitted for publication a new observation of this particle using a different decay channel.

The Ξ_cc⁺⁺ was first observed via its decay into a Λ_c⁺ baryon and three lighter mesons K^-, π⁺ and π⁺, with the Λ_c⁺ baryon decaying in turn into a proton p, a K^- meson and a π⁺ meson. The Ξ_cc⁺⁺ has now been "re-discovered" using a different decay, Ξ_cc⁺⁺→ Ξ_c⁺π⁺ (see graph), in which the Ξ_c⁺ baryon decays into a proton, a K^- meson and a π⁺ meson.

The left image shows the Ξ_c⁺π⁺ invariant mass distribution. The Ξ_cc⁺⁺ peak is clearly visible, with a statistical significance of 5.9σ against the background-only hypothesis. The mass peak represents an independent observation of the Ξ_cc⁺⁺ baryon, since the selected events are entirely different from those used in the previous study. The measured Ξ_cc⁺⁺ mass is 3620.6±1.5±0.4±0.3 MeV/c², fully consistent with the value of the previous measurement.

This measurement provides important information towards an improved understanding of the decays of doubly charmed baryons. Read more in the LHCb paper, and also in the CERN Update and the CERN Courier article.

21 November 2019 update: see Precision measurement of the Ξ_cc⁺⁺ mass paper.

12 June 2018: HeRSCheL looks even further forward. First physics results with the HeRSCheL detector.

In a typical proton-proton collision at the LHC, proton constituents, quarks or gluons, interact and move to opposite directions. They are interconnected by a strong interaction coloured field, a so-called "string". (Note an analogy to the electromagnetic field interconnecting electric charges.) The strength of this interaction increases with distance until finally the string breaks (fragments) into many hadrons collimated into two or more directions forming in this way characteristic jets. The remaining proton fragments, made of quarks or gluons which did not interact, continue to move in the direction close to their original trajectory, also carrying colour quantum numbers. Therefore, a coloured string interconnecting the two proton remnants is also formed, which then fragments into a high number of particles. The distribution of these particles is close to flat in the (pseudo)-rapidity η, η=-ln(tan(Θ/2)), being Θ the angle between the particle trajectory and the beam line.

Sometimes LHC protons also scatter at small angle without producing additional particles. This kind of interactions is interpreted as being produced by the exchange of an object named pomeron, which has quantum numbers of the vacuum. This concept was introduced in the Regge theory of strong interactions, a very popular theory which enjoyed a period of interest in the 1960s, and was then largely superseded by QCD.

Pomerons, emitted by each proton, can also interact, producing in turn a small number of hadrons. Since pomerons are colourless, rapidity regions depleted of accompanying particles appear, named “rapidity gaps”. This kind of interaction can also be observed if one of the pomerons is replaced by a photon. In the QCD language, a pomeron is interpreted as a two-gluon exchange in which no overall colour charge is transmitted. The left image shows a set of possible Feynman diagrams producing a J/ψ meson with or without a small number of accompanying particles. This kind of processes is referred as a Central Exclusive Production (CEP). The left image below shows a spectacular example of CEP in which two muons from a Υ(1S)-meson decay are reconstructed by the LHCb detector without any other signal recorded.

The High Rapidity Shower Counters for LHCb (HeRSCheL) detector was built to enhance studies of this interesting physics. It is located in the LHC tunnel at a maximum distance of 114m, on either side of the LHCb interaction point (see right image above). The detector was built in 2014 and installed at the beginning of 2015. It consists of twenty square scintillator modules, each about 30x30 cm² wide, in which tiny flashes of light are produced when they are traversed by charged particles. The scintillators are placed within centimetres of the LHC beam, just outside the vacuum pipe (see images below and video). They can therefore be used to detect particles produced by collisions at the LHCb interaction point, whose deviation from the beam direction is so small that they are not detected in the main LHCb apparatus but escape down the beam-pipe and only emerge further along the tunnel, near the HeRSCheL detectors. Somewhat counter-intuitively, then, a proton-proton interaction of interest is one that leads to nothing being detected by HeRSCheL! Indeed, these rare occurences correspond to CEP events where no particles are produced, except those from pomeron interactions. Thus absence of signal in HeRSCheL enables to be identified and removed very efficiently the much more abundant proton-proton interactions producing a significant number of hadrons.

The HeRSCheL detector is named after Caroline and William Herschel who, together, made great advances in the field of astronomy during the late 18th and early 19th centuries. Not unlike the work of the Herschel family, the HeRSCheL detector brings together well-known and well-established technologies in a novel application. In so doing, HeRSCheL provides a valuable extension to the LHCb physics programme.

Today the LHCb Collaboration submitted for publication first results of J/ψ and ψ(2S)-meson CEP in pp collisions at 13 TeV. The use of HeRSCheL allowed backgrounds and systematical uncertainties to be reduced significantly. Two muon tracks were clearly identified in the LHCb detectors. Events having additional activity, either in the form of charged or neutral particles observed in the LHCb detector as well as those with significant deposits in the HeRSCheL detector, were removed. The image shows clearly visible enhancements at the mass values of J/ψ and ψ(2S) mesons. The results are in agreement with theoretical calculations of two-meson production mainly through pomeron-photon interactions, the continuum being produced via photon-photon interactions. These results represent an excellent test of QCD, as well as an investigation of the nature of the pomeron, and also a means for constraining the gluon distribution in the proton (PDF) at low values of proton momentum fraction.

Read more in the LHCb paper, in the HeRSCheL performance paper and in the CERN Courier article.

25 May 2018: Long life(time) of the exceptionally charmed particle Ξ_cc⁺⁺. [ τ(Ξ_cc⁺⁺) = 0.256^+0.024_-0.022±0.014 ps ]

Last year the LHCb Collaboration announced the observation of an exceptionally charmed particle, the Ξ_cc⁺⁺ baryon. This contains two charm quarks and one up quark, resulting in an overall doubly positive charge. The Ξ_cc⁺⁺ baryon was identified via its decay into a Λ_c⁺ baryon and three lighter mesons K^-, π⁺ and π⁺ (see graph), with the Λ_c⁺ baryon decaying in turn into a proton p, a K^- and a π⁺ meson. Following this observation, LHCb is now undertaking precision studies of the properties of this special particle. These studies are made possible by the high production rate of heavy quarks at the LHC and the unique capabilities of the LHCb experiment, which can identify the decay products with excellent efficiency and purity. The precise measurements of the track trajectories by the LHCb Vertex Locator (VELO) enable the reconstruction of the proton-proton collision point, marked as “PV”, and of the decay points of the Ξ_cc⁺⁺ and Λ_c⁺ baryons, as seen in the cartoon.

This week, at the 9^th International Workshop on Charm Physics, CHARM 2018, in Novosibirsk, Russia, the LHCb Collaboration has presented the first measurement of the lifetime of the Ξ_cc⁺⁺ baryon. The data sample and the event selection are similar to those used in the analysis of the Ξ_cc⁺⁺ discovery. The experimental technique is based on the measurement of the decay time distribution relative to that of another decay with a similar topology, Λ_b⁰ → Λ_c⁺π^-π⁺π^-. As the lifetime of the Λ_b⁰ is already known with high precision from previous measurements, once the ratio of efficiencies for reconstructing the Ξ_cc⁺⁺ and Λ_b⁰ decays is determined, it is possible to derive the lifetime of the Ξ_cc⁺⁺ baryon. This approach allows for significant reductions of the systematic uncertainties associated to the measurement. The image shows the background-subtracted decay time distribution of the reconstructed Ξ_cc⁺⁺ baryons marked as “Data” points. The “Fit” distribution is created with the lifetime parameter adjusted to describe (fit) the data in the best possible way. The lifetime value obtained from the fit is 0.256^+0.024_-0.022±0.014 ps.

The measurement of the lifetime is critical to establish the nature of the Ξ_cc⁺⁺. The result clearly confirms predictions that it would have a relatively long lifetime value, which is a distinctive feature of weak interactions. However, it is very difficult to provide precise theoretical predictions, and thus they span a wide range, between 0.200 and 1.050 ps, depending on the phenomenological model that is used. The value measured by LHCb is within this range, but close to the lower end.

Read more in the LHCb presentation, in the CERN update in English and French, in the LHCb paper, and also in the CERN Courier article.

18 May 2018: New results presented at the Quark Matter Conference.

The activities of the LHCb collaboration have expanded far beyond the original core aims. The excellent performance of the detector has allowed the experiment to make important contributions to a wide range of research sectors becoming in this way a general purpose detector in the forward region. The results presented this week at the Quark Matter 2018 conference in Venice, are a perfect example demonstrating such extended LHCb capabilities. In addition to measurements in the “standard” lead-lead ion (PbPb) and proton-lead ion (pPb) LHC collision conditions, LHCb has been able to perform measurements with data taken in a unique configuration: a fixed-target operational mode. In fact, the LHCb vertex locator (VELO) was built with the possibility of injecting gas at very-low pressure into the interaction region, allowing for recording collisions of the LHC circulating proton beams with target nuclei at rest. This mode has been so far succesfully operated with He, Ne and Ar gases.

The LHCb collaboration presented a wide range of results on charm production in various types of collisions. These include the production of the Λ_c baryon in pPb collisions and of D⁰ and J/ψ mesons in the fixed-target collisions with He and Ar. The production of heavy quarks in nucleus-nucleus interactions is well suited to the study of the transition between ordinary hadronic matter and the hot and dense Quark-Gluon Plasma (QGP). The production of J/ψ mesons in nucleus-nucleus interactions, its possible suppression in the quark-gluon medium and/or later charm-anti-charm quark recombination are all studied in order to shed light into the mechanisms governing such a phase transition. The LHCb pPb and fixed-target results utilising proton interactions with different nuclei at different energies provide precious reference results in conditions in which the formation of the QGP is not expected. The images above show a signal mass peak of Λ_c baryons decaying into a proton, a K and a π (above left) as well as of D⁰ mesons decaying into a K and a π (above right).

In ultra-relativistic heavy nuclei PbPb collisions, two-photon and photonuclear interactions are enhanced in ultra-peripheral collisions (UPC). The collisions are either coherent, where the photon couples coherently to all nucleons, or incoherent, where the photon couples to a single nucleon. In the case of coherent J/ψ production in UPC, the photon-lead interaction can be modelled by the exchange of a colourless propagator, identified as a single object called a Pomeron, that interacts with the photon. The LHCb collaboration reported at the conference the cross-section measurement of coherent J/ψ production in PbPb collisions at 5 TeV and compared this to predictions from different phenomenological models. The image shows that the coherent J/ψ production (blue line) can be clearly separated from the other contributions in the natural logarithm of the J/ψ transverse momentum squared distribution.

Bound states of heavy quark and antiquark, such as the charmonium J/ψ and ψ(2S) as well as the bottomonium Upsilon particles, are very important tools to study properties of the QGP. It is expected that the experimentally observed rate of different bound states should be modified depending on the temperature of the QGP. Therefore the measurement of charmonium and bottomonium suppression can be used as a kind of QGP thermometer. The left image above shows different Upsilon states measured in pPb collisions at √s=8.16 TeV as a clear example that the LHCb detector is very well armed to study this interesting physics sector owing to its excellent particle identification and mass resolution.

High-energy collisions involving ions have the best chance to produce gluon condensates, where the gluon wave functions start to overlap producing a collective behaviour. This condensate would be similar to the phenomenon predicted by Bose and Einstein 93 years ago and observed in other boson systems such as ultra-cold atoms. Saturated gluons are expected to be observed only at small angles relative to the beam axes, where the number and the size of the gluons are the largest. LHCb has the unique capability of measuring photons coming from these high density gluon regions. The announcement of initial measurements of these photons caused a lot of excitement at the Quark Matter conference. The image above (right) shows the angular distribution between isolated photons and other particles taken during the 2016 pPb run. This sample is from a region where theorists expect that gluons are saturated. The peak at the angle π indicates the presence of photons from gluons. The blue band is the background from other processes. This is the first indication that gluons can be probed in this region, never achieved by any experiment so far. Members of the theory community expressed interest in studying the upcoming LHCb results and are discussing the mathematical tools that can confirm the discovery of this new form of matter.

Read more in the LHCb presentations [1], [2], [3], [4] and [5]. The LHCb papers and conference contributions will be available shortly.

28 April 2018: Start of 2018 data taking period.

The 2018 data taking period started officially today. The last 2017 proton-proton collisions took place on 28 November and the LHC machine was shut down during the winter period to allow for planned technical interventions. LHCb used this period to perform maintenance work on many sub-detectors.

Four months later proton beams passed again through the LHCb detector. The nominal proton beam has size of a human hair and an energy equivalent to a very fast train. The commissioning of LHC is therefore very complicated and takes about one month. First collisions with very low intensity beams took place on 12 April and the LHC experts were able to declare “Stable Beams” condition on 17 April. Once the “Stable Beams” condition is declared, the LHC operators keep the well focussed colliding beams on their stable orbits, which allows the experiments to switch on their sensitive detectors and start to safely record proton-proton collisions. Each beam consists of packets of protons called bunches. The number of proton bunches was progressively increased and will finally reach the maximum of 2556 bunches of which 2332 will collide inside the LHCb detector. The low intensity collisions are used for detector commissioning and therefore it is not evident at which moment experiments can call the official “start of data taking” used for physics analysis. The LHC operation team and LHC experiments agreed this year that this important milestone will be reached when 1200 proton bunches in each beam will collide. This has happened today.

The re-commissioning of the accelerator has proceeded very smoothly and first collisions arrived earlier than initially expected. The LHCb detector and its data acquisition system are ready for the last year of Run 2 data taking that will allow the experiment to obtain even more precise and interesting physics results. Follow LHCb data taking by watching live event display as well as live LHC and LHCb status pages. The image displays a typical event recorded today. The two-year Long Shutdown 2 will then start in December 2018, and during this period the LHCb detector will face its first major upgrade, which will allow the experiment to take data at much higher rate.

Click the image for higher resolution.

1 April 2018: Observation of a new particle consistent with long-hunted Eggs ball. Update 2 April 2018: see explanation here.

Today, at the Rencontres de Cuisine, the LHCb collaboration announced the observation of a new particle consistent with the properties of the long-hunted Eggs ball, η_gg, the smallest lump of nuclear force, predicted more than 40 years ago by Peter Eggs. Today is thus an eggstra special day in the history of particle physics.

In 2012 Atlas and CMS discovered the Higgs boson giving mass to other fundamental particles. Another important property of elementary particles is called flavour. The Eggs particle is responsible for the generation of flavour a few minutes after the Big Bang. Without the eggsistence of the Eggs particle all food would now taste the same. By studying the production and the decay properties of this new particle, LHCb physicists confirmed that it is composed of two eggs, that carry the strong sticky force, and hence it is really an egg ball, or more technically an egg-prolate-spheroid. The eggsistance of Eggs balls was predicted more that forty years ago, but it is only today that its reality is unambiguously confirmed eggsperimentally. In this way, the astonishing prediction of Peter Eggs about the strong interaction origin of flavour is confirmed. This idea is not implemented in the Standard Model of particle physics and therefore today’s discovery represents an eggstraordinary manifestation of the eggsistance of New Physics.

The image shows an eggcited theoretical physicist explaining properties of the newly discovered Eggs ball. The egg-like shape of η_gg is clearly visible.

The discovery was possible thanks to the excellent performance of the LHCb detector and the whole CERN accelerator system. The LHCb collaboration produced the η_gg by colliding high energy heavy ion beams: a small region of extremely hot nuclear matter was generated, which allowed the coagulation of the η_gg. The way to the grand unification between subatomic physics and molecular gastronomy is now therefore cracked wide open.

Read more in the CERN Update in the English and the French.

22 March 2018: Interesting results presented at the Rencontres de Moriond.

The LHCb Collaboration presented new interesting results at the Rencontres de Moriond EW and at the Rencontres de Moriond QCD. A few selected items are listed below.

(1) Search for a dimuon resonance in the Υ mass region. The only known fundamental scalar particle is the Higgs boson with a mass of 125 GeV. However, additional spin-0 particles, called Φ bosons, arise in many extensions of the Standard Model (SM) and are often predicted to be lighter than the Higgs boson. The LHCb detector has good sensitivity to light spin-0 particles produced by gluon-gluon fusion and decaying to a pair of opposite-sign muons, due to its capability of triggering on objects with small transverse momentum and to its high-precision spectrometer. The results of a search for these particles produced in proton-proton collisions at center-of-mass energies of 7 and 8 TeV were presented. No evidence is found for a signal in the mass range 5.5 to 15 GeV and upper limits are placed on the product of the production cross-section and the branching fraction to pairs of muons. The limits are competitive with the most stringent ones over most of the mass region considered, and are the first limits set near the Υ resonances. The image shows the mass spectrum of the muon pair in the whole search region. Mass peaks for five hypothetical Φ-boson mass hypotheses are displayed in green.

(2) First evidence for the decay B_s⁰→ K^*0μ⁺μ^-. First evidence for the decay B_s⁰→ K^*0μ⁺μ^- with a significance of 3.4 standard deviations was presented. The left image shows the invariant mass spectrum with the dominant peaking component of the decay B⁰→ K^*0μ⁺μ^-, while the zoom at the right is shown to emphasise the B_s⁰→ K^*0μ⁺μ^- contribution. This decay is predicted to be very rare within the SM, as it occurs only through suppressed loop diagrams. New particles foreseen in extensions of the SM can significantly enhance (or suppress) the rate of this decay. The result presented at the Rencontres de Moriond paves the way to search for new physics using this decay when a larger datasets will be collected by the upgraded LHCb detector during LHC Run-3.

(3) Measurement of forward top pair production. The production of top quarks at hadron colliders represents an important test of the SM. The top quark is the heaviest known fundamental particle and its production and decay properties are sensitive to a number of parameters involved in physics models beyond the SM. The unique forward acceptance of the LHCb detector allows a phase space inaccessible to general purpose detectors such as ATLAS and CMS to be probed. Top-quark production in this region receives a higher contribution from quark-antiquark annihilation than in the central region. In addition, it probes the the proton parton distribution functions (PDFs) at high values of the fraction of proton momentum carried by quarks or gluons, whose knowledge suffers from large uncertainties. At present precise measurements of top quark production at LHCb can be used to reduce substantially the uncertainty on PDFs in this kinematic region. The large contribution from quark-initiated production also results in a larger expected charge asymmetry in the forward region than in the central region probed by ATLAS and CMS.

Top pair production was measured at the proton-proton collision energy of 13 TeV, where the production cross-section in the acceptance of the LHCb detector is expected to be ten times larger than it was at the lower Run-1 energies. The higher cross-section allows LHCb physicists to select a high-purity sample, in which the dilepton (μ and e) channel is partially reconstructed by requiring that a muon (μ), an electron (e) and a b-jet are present in the event. The image compares the measured cross-sections within the LHCb acceptance with predictions of theoretical models. Read more in CERN Courier article.

(4) Measurement of CP violation in B⁰→D^∓π^± decays. Decay-time-dependent CP asymmetries in B⁰→D^∓π^± decays have been measured for the first time at a hadron collider by analysing the decay rate as a function of the decay time of B⁰ mesons. The image shows an example of the measured asymmetry as a function of the decay time of the B⁰. The parameters describing the difference in behaviour between matter and antimatter, known as CP violation, are constrained in the so called Cabibbo-Kobayashi-Maskawa matrix unitarity triangle. The angles of this triangle are denoted α, β and γ, and among these, γ is the least precisely known, see introduction in the news of 5 October 2012. The measurement of the asymmetries presented for the first time constrain the angle γ to an interval that is consistent with the current world average. The blue line in the image is the result of this analysis, while the red, dashed line shows the expectation in the absence of CP violation. An interesting possibility is to measure precisely the angle γ in processes where new physics contributions are in principle possible and in processes where this is not allowed, looking for differences. This result belongs to the class of measurements in which the contribution of new physics is not expected.

(5) Time-dependent and time-independent CP violation in B_(s)⁰→hh. The study of the CP violation in decays of B⁰ and of B_s⁰ mesons into two charged particles (h) that do not contain charm quarks represents a powerful tool to test the CKM picture of the SM, and also to investigate the presence of physics beyond the SM. The results of measurements of the time-dependent CP asymmetries in B⁰→π⁺π^- and B_s⁰→K⁺K^- decays as well as of the time-integrated CP asymmetries in B⁰→K⁺π^- and B_s⁰→π⁺K^- decays were presented. The results are the most precise from a single experiment and constitute the strongest evidence for time-dependent CP violation in the B_s⁰ meson decays to date. They also contribute to the determination of the CKM unitarity triangle. The image shows the time-dependent asymmetries for the decays (left) B⁰→π⁺π^- and (right) B_s⁰→K⁺K^-.

(6) Measurement of CP violation in the B_s⁰→φφ decay. The B_s⁰ decay into two φ mesons proceeds predominantly via a gluonic loop (penguin) diagram and, therefore, provides an excellent tool to search for new heavy particles which could appear within these loops, see the 14 June 2013 news for an introduction. A measurement of the time dependent CP-violating asymmetry in the B_s⁰→φφ decay was presented. The results obtained are consistent with the hypothesis of CP conservation. Each φ meson is observed through its decay into a K⁺K^- meson pair. Therefore, the B_s⁰ meson decay is visible in the invariant mass spectrum of four K mesons, see the red dashed line contribution in the image. At lower mass values, the contribution of the B⁰→φφ decay (green dotted line) is not large enough to be clearly observed yet, and the most stringent limit on its branching fraction was also presented.

Read more details in the LHCb Moriond EW presentations [1], [2], [3], [5], in the Moriond QCD presentations [2], [3] and [4,5,6] as well as in the LHCb papers and conference notes [2], [3].

28 November 2017: “Grand Unification” of data taking. End of data taking for 2017.

The 2017 data taking period ended this Sunday. Towards the end of the 2017 run at the centre-of-mass energy of 13 TeV, the LHC provided collisions at a reduced energy of 5 TeV to produce reference data for proton-lead and lead-lead collisions taken earlier in Run 2. Besides the scientific interest of proton-proton (p-p) physics at 5 TeV for the LHCb heavy-ion programme (see [1], [2]), the experiment has been taking at the same time a parallel stream of data from fixed-target collisions with another world first in high energy physics.

There have been typically 1836 bunches of protons circulating in each LHC ring, out of which 1094 collided inside the LHCb detector. LHCb physicists decided to use additional non-colliding bunches to accumulate the largest sample of proton-neon data in a fixed-target configuration. The LHCb experiment has the unique ability of injecting gas, neon in this case, into the interaction region and therefore study processes that would otherwise be inaccessible. This gas-injection system was originally designed to help LHCb measure the brightness of the accelerator's beams, but is now being used for dedicated physics measurements. This kind of operation is called by physicists a “fixed-target” mode in contrast to the standard “collider” mode used at the LHC, as in this case the LHC protons are colliding with stationary neon nuclei.

It has been the first time ever that an experiment has collected data in the collider and fixed-target modes simultaneously. LHCb physicists showed that it is possible to reconstruct both sets of data in parallel, align the detector elements and track particle trajectories correctly. A real challenge has been to develop an online event selection (trigger) system handling efficiently both data taking conditions. The live images (left) obtained by the data acquisition computer programs show reconstructed μ⁺μ^- invariant mass spectra. The J/ψ-meson peaks are clearly visible in the two different operational modes. The two-dimensional plot shows the z coordinate (along the proton beam direction) of the origin of the μ⁺μ^- pair. A strong accumulation around z=0 indicates the p-p collision point. The pink-dashed rectangle highlights the regions were p-p collision events were selected. The two other (red-dashed) rectangles show the region where only p-Ne collisions take place.

LHCb continues to revolutionise data acquisition and analysis techniques. Already two years ago the concepts of “online” and “offline” analysis were unified. The calibration and alignment process takes place now automatically online and stored data are immediately available offline for physics analysis. This time the collider and fixed-target modes of operation have been unified into the same data acquisition framework. In particle physics, a grand-unified theory is one in which at very high energies the electromagnetic, weak and strong interactions unify as a single force. Today LHCb physicists have succeeded to unify very different concepts of data taking and analysis.

The 2017 data taking period has been very successful, because of the excellent performances of both the LHC and the LHCb experiment itself. The image shows the growth of integrated luminosity during different years of LHC operation. The 2017 integrated luminosity is higher than that collected in 2016. The overall Run 2 luminosity (2015-2017), 3.7 fb^-1, is already higher than that recorded in Run 1 (3 fb^-1, 2010-2012).

A traditional end-of-year shutdown period, so-called Year End Technical Stop (YETS), is starting now. It will be used for maintenance and improvements to the LHC and its detectors. LHCb plans to exploit this period to perform maintenance work on many sub-detectors. It is planned that protons will start to circulate again in the LHC rings at the beginning of April 2018 and that the first p-p collisions for physics will take place in early May, marking the beginning of the last year of Run 2. The two-year Long Shutdown 2 will then start in December 2018, and during this period the LHCb detector will face its first major upgrade, which will allow the experiment to take data at much higher rate.

Read more in the CERN update.

12 November 2017: Implications of LHCb measurements.

This week LHCb physicists met with the theory community in the 7^th yearly workshop “Implications of LHCb measurements and future prospects”. The purpose of the meeting is to consider the latest results from LHCb, discuss possible interpretations and identify important channels and observables to test leading theoretical frameworks in the near and long-term future of the experiment.

The LHCb detector is used to perform precision measurement across a range of areas of particle physics, notably including searches for new physics manifestations in flavour, studies of heavy-flavour spectroscopy and production of gauge bosons, searches for new exotic particles, and unique measurements with heavy ion and fixed-target collisions in the forward region. Precise calculations from theory are essential for the interpretation of LHCb results, as is evident from numerous news articles on this page. The comparison of LHCb results with precise Standard Model predictions provided by the theory community is the key to looking for discrepancies that would indicate the existence, and the nature, of physics beyond our present knowledge. This series of joint LHCb-theory workshops is aimed at facilitating informal discussions between LHCb experimentalists and theorists, leading to a mutual exchange of information that is valuable for achieving progress.

More than 300 physicists crowded the Main Auditorium through three days featuring various dedicated sessions (see the photo above). The left image shows the steadily increasing number of participants as a function of time since the first workshop was held, clearly demonstrating that this series of workshops is more and more becoming a reference event for both the LHCb physicists and the theorists. Furthermore, intriguing results published by LHCb in recent years have been triggering additional interest and discussions within the physics community.

Learn more in the workshop presentations. Click the images for higher resolution.

13 October 2017: First xenon-xenon collisions at the LHC.

Yesterday evening the first xenon-xenon ion collisions took place at LHC and the run lasted for seven hours. The left image below shows the corresponding LHC control screen announcing the “Stable Beam” conditions for the data taking. The right image shows a collision event recorded and analysed online by the LHCb data acquisition system.

The images below show the invariant mass distribution of J/ψ mesons decaying to two muons and that of charmed D⁰ mesons decaying to K and π mesons promptly reconstructed by the LHCb software trigger. Heavy ion collisions are studied at LHCb in order to understand the behaviour of the so-called quark-gluon plasma, a state of matter in which quarks and gluons are moving as free particles, similarly to what happened in the first instants of time after the Big Bang. At the LHC, the properties of the quark-gluon plasma are usually studied in collisions of lead nuclei. Although there are no plans to use xenon-xenon collision for this purpose in the near future, one day of the LHC time was devoted to these collisions in order to study the properties of nuclear matter at high-energy density and high temperature. The comparison of experimental measurements in lead-lead and in xenon-xenon collisions will bring new insights into the properties of the quark-gluon plasma.

Read more about the LHCb’s heavy ion collision studies in this page [1], [2], [3], as well as in the CERN update on the xenon-xenon run in English and French.

13 September 2017: First test of lepton universality using charmed-beauty meson decays. Measurement of R(J/ψ): the ratio of branching fractions between B_c⁺→J/ψτ⁺ν_τ and B_c⁺→J/ψμ⁺ν_μ using τ⁺→ μ⁺ν_μ ν_τ decays. [R(J/ψ) = 0.71±0.17±0.18]

Today, at the open session of the Large Hadron Collider Committee, LHCC, the LHCb collaboration presented the result of a new measurement of the ratio of branching fractions, R(J/ψ), between B_c⁺→J/ψτ⁺ν_τ and B_c⁺→J/ψμ⁺ν_μ using τ⁺→ μ⁺ν_μ ν_τ decays. The result that was reported is approximately 2 standard deviations from the Standard Model (SM) expectation. Furthermore, an evidence of about 3 standard deviations for the semileptonic decay of charmed-beauty mesons including τ leptons was announced for the first time.

This result must be interpreted in the global picture of extensive tests of so-called “lepton universality”. This nomenclature comes from the fact that in the SM all charged leptons, such as taus (τ) or muons (μ), interact with identical strength (or, in physicists’ language, have the same "couplings"). However, mass differences between various leptons play a role which must be accounted for when performing calculations of interaction rates. In particular, the τ is much heavier than the μ lepton, and this leads to a SM prediction for the ratio R(J/ψ) substantially smaller than unity, between 0.25 and 0.28.

A programme of measurements of lepton flavour universality with τ leptons in the final state is being performed by the LHCb, BaBar and Belle collaborations. The most recent results were reported by LHCb for R(D*), the ratio of branching fractions between B⁰→D^*-τ⁺ν_τ and B⁰→D^*-μ⁺ν_μ, see the 6 June 2017 news. B⁰ and B_c⁺ mesons differ only by a quark accompanying the b quark inside a B⁰ or B_c⁺ meson, as visible in the image above. Any new physics contribution to the decay diagram as for example those sketched in the image would affect both R(D*) and R(J/ψ) ratios in a similar way.

The R(J/ψ) measurement is very challenging. Due to presence of ν leptons invisible in the detector, both decays, involving either τ or μ leptons, are observed only through the three muons. Two of them are perfectly identified as arising from the decay of a J/ψ meson. The third μ is the key addition to enable semileptonic B_c⁺ decays involving τ or μ leptons to be distinguished from the background.

The left hand side image shows the summary of R(D*) measurements as presented in the 6 June 2017 news. It is interesting to notice that different experiments operating either at pp (LHCb) or e⁺e^- (BaBar, Belle) colliders, using very different experimental techniques, measure values systematically above the SM prediction. The value of R(J/ψ), 0.71±0.17±0.18, reported today and presented in the lower part of the image, is also lying above the SM prediction. (Note the different horizontal scale.)

LHCb has also found other intriguing anomalies when performing tests of lepton universality. The theoretically very clean ratios R(K) and R(K*) both show deviations from the SM prediction at the level of about 2.5 standard deviations. All these measurements were performed at LHCb using the entire Run 1 data sample, corresponding to an integrated luminosity of 3fb^-1 at centre-of-mass energies of 7 and 8 TeV. Data collected in Run 2 already provide a sample of B-meson decays more than twice as large, and it will be of great importance to see whether future updates of these analyses with increased statistics will confirm the hints of discrepancies. The ability of LHCb to perform measurements using different b-hadron species, notably including the B_c meson in this case, but also the B_s meson and the Λ_b baryon in the forthcoming future, will play a crucial role in order to clarify the global picture of deviations from lepton flavour universality that is emerging.

Read more in the LHCb presentation, in the CERN Courier R(j/ψ) article, in the CERN Courier April 2018 review article, and in the LHCb publication.

13 September 2017: New avenue to precision mass measurements of the χ_c1 and χ_c2 mesons. [m(χ_c1) = 3510.71±0.04±0.09 MeV; m(χ_c2) = 3556.10±0.06±0.11 MeV] [m(χ_c2) - m(χ_c1) = 45.39±0.07±0.03 MeV] [Γ(χ_c2) = 2.10±0.20±0.02 MeV]

Today, at the open session of the Large Hadron Collider Committee, LHCC, the LHCb collaboration presented the result of a precise mass measurements of χ_c1 and χ_c2 mesons, performed for the first time by utilising the newly discovered decay χ_c1,2→μ⁺μ^-J/ψ.

χ_c1, χ_c2 and J/ψ mesons belong to a family of particles that are commonly referred to as charmonium states. They are bound systems of a charm quark, c, and an anti-charm quark, c, held together by the strong nuclear force. Just like ordinary atoms, e.g. hydrogen, c and c quarks can be arranged in various quantum states with different angular momenta and spin configurations, giving rise to a spectrum of particles with different masses, all composed of the same fundamental quarks. The image on the left represents the complexity of this spectrum along with some of the allowed decay transitions as a function of mass (in the vertical axis) versus spin and other quantum numbers (in the horizontal axis). The decay paths corresponding to the decays under study are highlighted in red. The first charmonium particle, the J/ψ meson, was discovered on the 11^th of November 1974. This discovery triggered rapid changes in high-energy physics at the time and these are commonly referred to as the "November Revolution". The charmonium family was then intensively studied by a large number of experiments at different facilities. In particular, the masses of χ_c states were measured by studying radiative decays into J/ψ mesons, i.e. χ_c1,2→γJ/ψ, and later by using dedicated experiments employing collisions of protons and antiprotons for their production. As it is experimentally very challenging to measure the energy of a photon precisely in the harsh environment of a hadron collider, high-precision measurements have not previously been made at a collider like the LHC.

The new analysis from LHCb applies an old "trick" in a new situation. This technique was first proposed in 1951 by R.H. Dalitz to study the decay of the π⁰ meson, which had been discovered one year earlier. Decays of π⁰ mesons into two real photons were not easy to observe in the detectors used at the time. R.H. Dalitz realised that quantum mechanical fluctuations permitted one (or both) of the photons to be virtual, allowing them to transform into an electron-positron pair.

LHCb physicists observed the new decay χ_c1,2→γ^*J/ψ where the virtual γ^* transforms into a μ⁺μ^- pair giving a rise to a χ_c1,2→μ⁺μ^-J/ψ decay. The high precision of the LHCb spectrometer allowed two narrow χ_c peaks to be observed in the invariant J/ψμ⁺μ^- mass with excellent resolution, as apparent in the right image above. The values of the masses of the χ_c1 and χ_c1 states, m, along with another important property of the χ_c2 called "natural width", Γ, are measured through this novel decay. The new measurements have a similar precision to and are in good agreement with those obtained at previous dedicated experiments, notably E760 and E835, that used antiproton annihilations into a hydrogen target to produce and study the charmonium states (see the image below for a comparison amongst the various results available in the literature). This new measurement opens a new avenue to precision studies of the properties χ_c mesons at the LHC, more than 40 years after the November Revolution took place. It will facilitate a better understanding of quantum chromodynamics (QCD), i.e. the theory that describes the behaviour of the strong nuclear force.

More information can be found in the LHCb presentation and in the corresponding LHCb paper. Read more in the CERN update for the public and in the CERN Courier article.

7 September 2017: 6 fb^-1: celebrating “Jeûne genevois”.

The LHCb collaboration has reached a symbolic milestone this morning, announcing that the experiment has recorded a luminosity of 6 fb^-1 integrated over the whole period of data taking 2010-2017. In this way LHCb celebrated ”Jeûne genevois”, a public holiday in the canton of Geneva, where CERN is hosted. The shift crew captured this moment by taking a photo of the live screen in the LHCb Control Centre, see the number 6000 (pb^-1) in the lower-left corner of the table in the image. The luminosity delivered by the LHC collider was 6599 pb^-1 during this period. LHCb recorded data with an impressive average efficiency of nearly 91%, as it can be seen in the table.

Most of the results highlighted through the years in this page were obtained using the LHC Run 1 data sample, where the full sample corresponds to an integrated luminosity of 3 fb^-1 at centre-of-mass energies of 7 and 8 TeV. Today’s announcement means that LHCb has recorded the same luminosity during Run 2 as at the end of Run 1, but now with centre-of-mass energy of 13 TeV. Since the production cross-section of beauty and charm particles at 13 TeV is about twice as large as that in Run 1, the total sample of beauty particles available for physics analyses is now about three times larger than that recorded during the Run 1 data taking period. The door that will lead to obtain many more interesting results is open wide, as you will be able to see with our future reports in this page.

15 July 2017: Several new results presented at the EPS-HEP conference.

The LHCb collaboration presented new results in 32 talks given by LHCb speakers at the EPS Conference on High Energy Physics, EPS-HEP 2017, Venice, Italy. The observation of an exceptionally charming particle, the Ξ_cc⁺⁺, was already announced in the 6 July 2017 news. A few other selected items are listed below.

(1) Prompt and nonprompt J/ψ production and nuclear modification in p-Pb collisions at √ s_NN  = 8.16 TeV. Proton collisions with lead nuclei (p-Pb) at the LHC are very important to understand nuclear effects relevant to the study of quark-gluon plasma formation in lead-lead (Pb-Pb) collisions. The analysis of J/ψ production is of particular interest. Two sets of data were taken: p-Pb and Pb-p, where in the first case the proton travels in the forward direction of LHCb and in the second case the beam directions are reversed. This allowed the LHCb detector, recording the particles only on one side of the interaction point, to make measurements in both forward and backward directions with respect to the proton beam. The LHCb collaboration already obtained results at a collision energy √ s_NN  = 5 TeV, see e.g. the news on 10 May 2013. New results presented at the EPS-HEP conference are obtained at √ s_NN  = 8.16 TeV with 10 and 40 times larger data sets in p-Pb and Pb-p collisions, respectively. Both J/ψ mesons produced directly (prompt) and originating from beauty-hadron decays (nonprompt) are studied. A suppression of prompt J/ψ production compared to pp collisions of up to 50% (25%) in p-Pb (Pb-p) at low transverse momentum is observed. For the first time, beauty-hadron production is measured precisely at low momentum at the LHC in p-Pb and Pb-p collisions. In p-Pb, a weak suppression is observed, whereas in Pb-p no significant deviation from unity is found. The LHCb measurements are very important to test and tune different theoretical models of J/ψ production in cold nuclear matter. The image shows how prompt J/ψ (blue dashed line) and J/ψ-from-beauty-hadron (purple solid line) productions are statistically separated in the analysis.

(2) Measurement of CP observables in B^± → D^(*)0K^± and B^± → D^(*)0π^± decays. World's best measurements of CP observables in B^± → D⁰h^± decays are obtained with the D meson reconstructed in the Kπ , KK and ππ final states. Measurements of partially reconstructed B^± → D^*0h^± decays (see the mass distribution in the left image below) are also reported for the first time. Measurements of CP observables in B → (D^*0→D⁰γ)K decays are competitive with the current world averages; the equivalent observables measured in B → (D^*0→D⁰π⁰)K decays substantially improve upon the existing world averages. Evidence for CP violation in B → (D^*0→D⁰π⁰)K decays is found with a statistical significance of 4.3 standard deviations. The combination of this results with all other available LHCb measurements of the γ angle of the unitarity triangle, γ = (76.8^+5.1_-5.7)°, yields quite a significant reduction of the overall uncertainty on this important quantity.

(3) Measurement of the forward Z → bb cross-section in pp collisions at a centre-of-mass energy of 8 TeV. Several extensions of the Standard Model predict that new heavy particles that decay to two energetic b-quarks could be accessible at the energy of LHC collisions. Moreover, the study of the Higgs-boson decay to a pair of b and b quarks at the LHC is of great interest, since the precise determination of the Higgs boson coupling to b-quarks is an important test of the Standard Model. The decay of a Z boson to a bb pair provides a standard candle for direct searches of physics beyond the Standard Model in final states with bb quarks. Measurements of this decay can be used to demonstrate that no biases are induced by the b-jet reconstruction procedure and that the reconstruction efficiencies are evaluated correctly. A clear signal is observed by LHCb for the first time, as shown in the right image above. The product of the cross-section and branching fraction is measured with a precision of about 20%. The theoretical prediction and the measurement are compatible within one standard deviation. Additional data being collected by LHCb will enable a more stringent comparison with the theoretical prediction to be performed. This result opens a new avenue of measurements at LHCb, in a domain typically studied by ATLAS and CMS and where LHCb was not expected initially to give significant results. This is a further example that illustrates that LHCb has become a general purpose forward experiment with a broad physics programme.

(4) Rarest B⁰ decay ever observed. The B⁰ meson decay into a pp pair is observed for the first time with a statistical significance of 5.3 standard deviations. The image shows the pp invariant mass spectrum where the contribution of B⁰→ pp decay is clearly visible. The branching fraction is measured to be (1.25 ± 0.27 ± 0.18)x10^-8. This decay is the rarest fully hadronic decay of a B meson ever observed. Studies of B mesons decaying to baryonic final states have been carried out since the late 1990s. It was quickly realized that baryonic B decays differ from mesonic decays since two-body charmless decays are suppressed with respect to decays to multi-body final states. This observation provides a valuable input towards the understanding of the dynamics of hadronic B decays and allows for a better scrutiny of QCD models. In the future, with the analysis of additional data, it is hoped that the B_s⁰→ pp decay will also be observed.

Please click the images for higher resolution versions. More details on the measurements are given in LHCb EPS-HEP conference presentations, these are linked from the titles of each section (written in italics).

6 July 2017: Observation of an exceptionally charming particle. [ m(Ξ_cc⁺⁺) = 3621.40±0.72±0.27±0.14 MeV/c² ]

Today, at the EPS Conference on High Energy Physics, EPS-HEP 2017, in Venice, Italy, the LHCb collaboration presented the first observation of a doubly charmed particle. This particle, called the Ξ_cc⁺⁺, is a baryon (particle composed of three quarks) containing two charm quarks and one up quark, resulting in an overall doubly positive charge. It is a doubly charm counterpart of the well-known lower mass Ξ⁰ baryon, which is composed of two strange quarks and an up quark.

The Ξ_cc⁺⁺ baryon is identified via its decay into a Λ_c⁺ baryon and three lighter mesons K^-, π⁺ and π⁺. The image above shows an example of a Feynman diagram contributing to this decay. The Λ_c⁺ baryon decays in turn into a proton p, a K^- and a π⁺ meson. The image shows the Λ_c⁺K^-π⁺π⁺ invariant mass spectrum obtained with 1.7 fb^-1 of data collected by LHCb in 2016 at the LHC centre-of-mass energy of 13 TeV. The mass is measured to be about 3621 MeV/c² which is almost four times heavier than the most familiar baryon, the proton, a property that arises from its doubly charmed-quark content. The signal candidates are consistent with particles that traveled a significant distance before decaying: even selecting only those Ξ_cc⁺⁺ particles that survived more than approximately five times the expected decay time resolution, the signal remains highly significant. This state is therefore incompatible with a strongly decaying particle, but is consistent with a longer-lived decay involving weak interactions as would be expected for this particle.

The existence of doubly charmed baryons was already known to be a possibility in the 1970s, after the discovery of the charm quark. In the early 2000s the observation of a similar particle was reported by the SELEX collaboration. This observation was not confirmed by subsequent experiments and the measured properties of this particle are not compatible with those of the Ξ_cc⁺⁺ baryon discovered by LHCb. The discovery of the Ξ_cc⁺⁺ performed by LHCb has been made possible by the high production rate of heavy quarks at the LHC and thanks to the unique capabilities of the experiment, which can identify the decay products with excellent efficiency and purity. The image shows an artist view of this new particle. This animation shows how the signal accumulated in the Λ_c⁺K^-π⁺π⁺ invariant-mass spectrum throughout 2016.

This discovery opens a new field of particle physics research. An entire family of doubly charmed baryons related to the Ξ_cc⁺⁺ is predicted, and will be searched for with added enthusiasm. The image illustrates how half-spin baryons can be formed by assembling together the three light quarks (u, d, s) and the charm quark (Particle Data Group, Phys.Rev. D86, 010001). Furthermore, other hadrons containing different configurations of two heavy quarks, for example two beauty quarks or a beauty and charm quark, are waiting to be discovered. Measurements of the properties of all these particles will allow for precise tests of QCD, the theory of strong interactions, in a unique environment. LHCb is very well equipped to face this very exciting challenge.

Click images for higher resolution. More information can be found in the LHCb EPS-HEP presentation, in the LHCb publication and soon in the CERN seminar. Read also the CERN Press Release in English and French as well as the CERN Courier article.

3 July 2017: Hélène Langevin-Joliot visited the LHCb detector cavern.

Hélène Langevin-Joliot visited today the LHCb detector cavern during a short LHC Technical Stop (left image). She is the Emeritus Research Director in Fundamental Nuclear Physics at CNRS in Orsay (France), and is the daughter of Frédéric and Irène Joliot-Curie (Nobel laureates for Chemistry in 1935) and granddaughter of Pierre Curie (Nobel laureate for Physics in 1903) and Marie Curie (Nobel laureate for Physics in 1903 and for Chemistry in 1911). During her visit to CERN she gave a lecture at the Globe of Science and Innovation entitled “Marie Curie, women and science, then and now”. A special musical performance was given in Thoiry, a nearby village to CERN, where she discussed the photograph shown here (right image).

The photo was taken in Thoiry in 1930, in front of the Hôtel Léger, during what we now call a conference dinner and included both Marie Curie and Albert Einstein. They were both members of the International Committee on Intellectual Cooperation, an advisory organization for the League of Nations which aimed to promote international exchange between scientists, researchers, teachers, artists and intellectuals. The committee went for dinner to the then famous restaurant in Thoiry on 25/7/1930 during its annual meeting. After World War II, the League of Nations was replaced by the United Nations and the International Committee on Intellectual Cooperation was a forerunner of UNESCO. An intergovernmental meeting of UNESCO in Paris in December 1951 led to the creation of the European Council for Nuclear Research, which was responsible for establishing the European Organization for Nuclear Research (CERN) in 1954.

1 July 2017: Giovanni Passaleva and Chris Parkes – new management for the LHCb Collaboration

Giovanni Passaleva from the Istituto Nazionale di Fisica Nucleare in Firenze begins today his 3-year tenure as LHCb spokesperson. He takes over from Guy Wilkinson from the University of Oxford. Chris Parkes from the University of Manchester is the new deputy spokesperson for this period, taking over from Monica Pepe Altarelli from CERN.

click the images for higher resolution

Giovanni and Chris will face the huge challenges of completing the run 2 data taking and preparing for the major LHCb detector upgrade to be installed during Long Shutdown 2, LS2. In the meantime they dream that the analysis of the Run 2 data could yield the discovery of new physics!

Guy and Monica – thank you for your excellent coordination in the past three years.

Giovanni and Chris – good luck.

Read more details at the LHCb Collaboration and CERN Webpages in English and French Web pages.

6 June 2017: New test of lepton universality. Measurement of R(D^*): the ratio of branching fractions between B⁰ → D^*-τ⁺ν_τ and B⁰ → D^*-μ⁺ν_μ using three-prong hadronic τ decays. [R(D^*) in this measurement = 0.291±0.019±0.029; LHCb average = 0.310±0.016±0.022]

Yesterday, at the 15^th Flavor Physics and CP violation conference in Prague (Czech Republic), and today at a seminar at CERN, the LHCb collaboration presented the results of a new measurement of the ratio of branching fractions R(D^*) = BF(B⁰ → D^*-τ⁺ν_τ)/BF(B⁰ → D^*-μ⁺ν_μ) using for the first time τ⁺→π⁺π^-π⁺ν_τ or τ⁺→π⁺π^-π⁺π⁰ν_τ decays. The D^*- meson was reconstructed through the D^*-→D⁰(→K⁺π^-)π^- decay chain. The result (*) 0.291±0.019±0.029 is characterised by the best statistical uncertainty among any single measurement performed so far, with a central value that is higher than the Standard Model (SM) prediction, although consistent with it at one standard deviation. It is the first time that R(D*) had been measured with these three charged pion decay modes of the τ lepton. This has been possible because of the unique capabilities of LHCb in precisely reconstructing decay vertices. This measurement is affected by different sources of systematic uncertainties, with respect to previously existing analyses, and opens a new avenue to precision studies in the sector of lepton universality with τ leptons. An average of this measurement with a previous LHCb determination using muonic τ decays, τ^-→μ^-ν_μν_τ, gives R(D^*) = 0.310±0.016±0.022, which is consistent with the world average and differs by 2.1 standard deviations from the SM prediction.

In the SM all charged leptons, such as taus (τ) or muons (μ), interact in an identical fashion (or, in physicists’ language, have the same "couplings"). This property is called "lepton universality". However, differences in mass between the leptons must be accounted for. In particular the τ lepton is much heavier than the μ lepton and this leads to a SM prediction for the ratio R(D^*) substantially smaller than unity. This ratio is precisely calculable in the SM owing to the cancellation of uncertainties in the ratio, and turns out to be about 0.252 with excellent precision.

The image shows a comparison of different measurements of R(D^*). It is interesting to notice that different experiments operating either at pp (LHCb) or e⁺e^- (BaBar, Belle) colliders, using very different experimental techniques, measure values systematically above the SM prediction. The average of all (world) results is brought, by including this new measurement, a little bit closer to the SM prediction and at the same time, due to improved precision, the discrepancy between the experimental world average and the SM prediction increases slightly to about 3.4 standard deviations.

Any measurement exhibiting a conclusive breakdown of lepton universality, after mass related effects are accounted for, would be a clear sign of new physics. The ratio R(D^*) is particularly interesting since a large class of SM extensions contains new interactions that involve the third generation of quarks and leptons, like a b quark (from a B hadron) and τ⁺ and ν_τ leptons in this case.

LHCb has also found other intriguing anomalies when performing tests of lepton universality. The theoretically very clean ratios R(K) and R(K*) both show deviations from the SM prediction of identical behaviour of muons and electrons at the level of about 2.5 standard deviations. Recently the CERN Theory Division organized a three-day workshop to discuss the interpretation and implications of these anomalies and their potential to shed some light into models of physics beyond the SM.

Both R(D^*) measurements were performed at LHCb using the entire Run 1 data sample, corresponding to an integrated luminosity of 3fb^-1 at centre-of-mass energies of 7 and 8 TeV. Data collected in Run 2 already provide a sample of B-meson decays more than twice as large, and it will be of great importance to see whether updates of the Run 1 analyses will confirm the discrepancy.

(*) Results updated in the April 2018 publication. More information can be found in the LHCb presentation at the conference, at the CERN seminar. Read more in the CERN update for scientists.

23 May 2017: Start of 2017 data taking period.

Today proton-proton collisions for physics restarted again after a 5 month “Extended Year End Technical Stop”. This traditional winter shut-down period was "extended" due to major installation work carried out in another one of the LHC detectors. The recommissioning of the accelerator has proceeded very smoothly and first collisions arrived earlier than initially expected. During the technical stop LHCb performed relatively minor maintenance work on the detector, and, on the other hand, major interventions on the access lift and the crane in the underground cavern. The LHCb detector and its data acquisition system are ready for a bumper year of data taking that will allow the experiment to obtain even more precise and interesting physics results. The image displays a typical event recorded today.

Click the image for higher resolution.

18 April 2017: Lepton universality test probes physics beyond the Standard Model. LHCb finds new hints of possible deviations from the Standard Model.

Today, at a seminar at CERN, the LHCb collaboration presented results on the measurement of R_K*⁰, which is the ratio of the probabilities that a B⁰ meson decays to K*⁰μ⁺μ^- and to K*⁰e⁺e^-. The K*⁰ meson was reconstructed through its decay into a charged kaon K⁺ and a pion π^-. This measurement provides an important test of lepton universality (LU), which is one of the most important ingredients of the Standard Model of particle physics. LU means that leptons (e.g., electrons e and muons μ) behave in the same way, i.e., they have the same couplings to gauge bosons.

According to the SM, therefore, R_K*⁰ is expected to be close to unity (small, well-understood effects related to the masses of the leptons prevent the parameter being exactly this value). In the LHCb measurement, the distance of the result from the SM prediction is found to be significant at the level of 2.1-2.5 standard deviations in each of the two regions of q² (the μ⁺μ^- or e⁺e^- invariant mass squared, see text below) in which the measurement is performed.

Any conclusive observation of LU violation would indicate the existence of physics beyond the Standard Model (BSM). Searches for BSM physics are listed among the most important goals of the LHC physics programme. The B⁰→K*⁰μ⁺μ^- and B⁰→K*⁰e⁺e^- decay rates could be affected by the presence of so-called heavy “virtual” BSM particles, which couple differently to electrons and muons than the gauge bosons, and therefore could sizeably increase or decrease the rate of the K*⁰μ⁺μ^- and K*⁰e⁺e^- final states, resulting in a deviation of R_K*⁰ from the SM prediction.

The image shows the measured value of R_K*⁰ in two regions ([0.045, 1.1], [1.1, 6.0] GeV²/c⁴) of the μ⁺μ^- or e⁺e^- invariant mass squared, q². The numerical values are given at the top of the article, where the first uncertainty, which dominates, is statistical, and the second is systematic. The lower boundary of the low-q² region roughly corresponds to the di-muon production threshold. The 6 GeV²/c⁴ upper boundary is chosen to reduce a possible contamination from the J/ψ particle. The image also shows several independent SM theoretical predictions. A difference from the SM of 2.1-2.3 standard deviations is observed in the low-q² region and of 2.4-2.5 in the high-q² interval. These differences are not yet at the level where they can be claimed to exhibit evidence for BSM physics, but they are intriguing when considered in the context of an earlier LHCb analysis.

In fact a previous LHCb measurement of the quantity R_K in which the B⁰ meson is replaced by a B⁺ and the K*⁰ meson by a K⁺ in the ratio, was also found to have a similar value and be lower than unity by 2.6 standard deviations. This result created much interest in the particle physics community.

What kind of BSM physics could explain the LHCb results? Examples are shown in the Feynman diagrams below. The upper ones show the SM contributions. The left-lower one shows a possible contribution from a heavy Z-boson-like particle, named Z', which would interact differently with muons and electrons. The lower-right diagram shows a possible contribution from a hypothetical scalar leptoquark LQ, which would interact with both quarks and leptons. Alternatively, different, not yet predicted, and therefore even more interesting, BSM physics could be at play!

The LHCb collaboration has a wide programme of LU tests based on different R measurements in which other particles replace the K*⁰ or K⁺ mesons in the ratios. The R_K*⁰ and R_K measurements discussed here are obtained using the entire Run 1 data sample corresponding to an integrated luminosity of 3fb^-1 at the LHC centre-of-mass energies of 7 and 8 TeV. Data collected in Run 2 already provide a sample twice as large, and it will be of great importance to see whether updates of the present analysis will confirm the discrepancy. In addition, another class of measurements concerning different ratios of B-meson decay rates involving τs and μs, also exhibit some tensions with the SM predictions. Future LHCb measurements will be able to elucidate whether these tantalising hints are a manifestation of statistical fluctuations or whether LHCb is observing a glimpse of new physics.

More information can be found in the LHCb seminar and in the LHCb paper. Read more in the CERN update for the public both in English and in French, in the CERN Courier article, in the Nature news and also in the Symmetry magazine.

27 March 2017: Measurement of antiproton production in p-He collisions. Can cosmic-ray antiprotons unveil dark matter collisions?

This week, at the 52^nd Rencontres de Moriond EW in La Thuile Italy, LHCb presented results of an analysis which may have significant consequences for the search for “dark matter” in the universe. The measurement is of antiproton production in proton-helium (p-He) collisions. Although the LHC collides protons with protons, the LHCb experiment has the unique ability to inject gas, for example helium, into the interaction region and therefore study processes that would otherwise be inaccessible, such as here the production of antiprotons from p-He interactions. The forward geometry and particle identification capabilities of the LHCb detector are well suited to provide good reconstruction for antiprotons down to the low transverse momentum region where most of the production is expected. This gas-injection system was originally designed to help LHCb measure the brightness of the accelerator's beams, but is now being used for dedicated physics measurements.

This result is very important for interpreting searches for dark matter in the Universe. Dark matter is a hypothetical entity of unknown nature whose existence would explain a number of otherwise puzzling astronomical and cosmological observations. The name refers to the fact that it does not interact with electromagnetic radiation (like light). Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motion of visible matter around galactic centres and precise measurements of temperature fluctuations in the cosmic microwave background. An interesting possibility is that dark matter is composed of some kind of stable elementary particles whose existence is proposed in different extensions of the Standard Model of particle physics. In such a case these dark matter particles could collide and produce ordinary particles, in particular antiprotons. However antiprotons can also be produced in standard processes through collisions of cosmic rays with the interstellar medium, of which helium is a significant component. Therefore a potential signature of dark matter is the observation in space of a higher ratio of antiprotons to protons than would be expected from standard processes. Excitingly, this tendency is hinted at in measurements from the PAMELA and AMS experiments, as seen in the image that shows the measured p/p ratio against energy. Also shown in the image is the prediction ('Fiducial') and, as coloured bands, the uncertainties on this prediction, which come from the limited knowledge of several of the ingredients in the calculation. Although the data points lie above the prediction, the current uncertainties are large enough to almost accommodate the discrepancy, thereby preventing an unambiguous interpretation. The largest uncertainty is associated with the knowledge of the cross-sections, in particular that of p-He collisions. This is where LHCb enters the game.

LHCb has performed the first measurement of the antiproton cross-section in p-He collisions in an energy range that is critical for the interpretation of the PAMELA/AMS-02 studies. A precision of around 10% is attained, which is significantly more precise than the assumptions that have entered the cosmic ray calculations. In the image, the result is compared with the most popular models used in cosmic rays physics. The spread among model predictions indicate the large uncertainty on the process prior to this measurement. It will be very interesting to see how this measurement affects the prediction of the p/p ratio in space, and from this, what are the consequences in the searches for dark matter.

More details can be found in the LHCb presentation and soon in the LHCb conference note. Read more in the CERN update for the public both in English and in French and also in the CERN Courier article.

16 March 2017: The magic of the Ω_c baryon. Observation of five new narrow Ω_c⁰ excited states.

As NASA announced a few days ago the discovery of the first system of seven Earth-size planets orbiting around a single star, so LHCb unveiled today the discovery of a new system of five particles, with the observation of five new narrow Ω_c⁰ excited states.

LHCb physicists reconstructed a highly pure sample of charmed baryons Ξ_c⁺, with quark content csu, decaying into a proton p, a kaon K^- and a pion π⁺. The p K^-π⁺ invariant mass spectrum is shown in the left image below. The red peaking distribution shows the Ξ_c⁺ contribution above the background indicated by the blue dashed line.

Subsequently the Ξ_c⁺ candidates were combined with K^- mesons present in the same event. The Ξ_c⁺ K^- invariant mass distribution obtained in this way is shown in the right image above, revealing for the first time five narrow structures with an overwhelming statistical significance. These structures are interpreted as manifestations of excited states of the Ω_c⁰ baryon. These excited states decay into a Ξ_c⁺ baryon and a K^- meson via the strong interactions, in contrast to the weak decays responsible for the three particles used to form the Ξ_c⁺ mass peak.

The Ω_c⁰ baryon is a higher mass partner of the Ω^- baryon, a particle which played a very important role in the history of particle physics. In the 1950s many different particles were discovered. Initially thought to be elementary, the ever growing list of discoveries led physicists to doubt this assumption. Therefore efforts were made to find a classification scheme in analogy to the periodic table of chemical elements. The most successful such scheme was proposed by Gell-Mann. In this model mesons and spin ¹⁄₂ baryons are organized into octets (Eightfold Way) while spin ³⁄₂ baryons form a decuplet, as displayed in the image on the left. When the scheme was proposed, the top-most particle in the image, the Ω^-, was not yet discovered, but hypothesised to exist in order to complete the pattern. The regular structure of the decuplet enabled many properties of this new particle to be predicted, including its mass. The Ω^- was subsequently discovered through a single famous bubble-chamber photograph obtained at the Brookhaven laboratory in 1964 (the corresponding line diagram is shown in the right picture). This picture validated the Eightfold Way, and led Gell-Mann to propose the quark model in 1964, which explains the structure of the octets and decuplet. In the quark model the Ω^- is composed of three strange quarks (sss). The Ω_c⁰ is like a Ω^-, but contains a charm (c) quark in place of one of the strange quarks.

The ground state of the ssc system, the Ω_c⁰ itself, has been known about for quite some time. The discoveries announced today are excited states of this system, analogous to the excited states of atoms. These states are named the Ω_c(3000)⁰, Ω_c(3050)⁰, Ω_c(3066)⁰, Ω_c(3090)⁰ and Ω_c(3119)⁰, where the numbers indicate their masses in MeV as measured by LHCb. By discovering these particles LHCb has reinforced the exceptional role of the Ω baryon family in the history of particle physics.

Click the images for higher resolution. More details can be found in the LHCb publication. Read more in the CERN update for the public both in English and in French and also in the PRL Synopsis.

11 March 2017: New results presented at the Rencontres de la Vallée d’Aoste.

The LHCb collaboration presented several new results at the 31^st Rencontres de Physique de la Vallée d'Aoste taking place this week in La Thuile, Italy. A few selected items are listed below.

Processes where a B meson decays into a pair of oppositely charged leptons are powerful probes in the search for physics beyond the Standard Model. Following up the preliminary result that was presented recently at a CERN seminar on the first observation by a single experiment of the decay B_s⁰→ μ⁺μ^-, with a statistical significance of 7.8 standard deviations, the final result was shown at La Thuile for the first time. This presentation provoked much interesting discussion. A search for the tauonic decays B→τ⁺τ^-, where B can be either a B⁰ or a B_s⁰ meson, was also shown in La Thuile. The τ leptons are reconstructed in the analysis through the decay τ→π^-π⁺π^-ν_τ. Decays of B⁰ and B_s⁰ mesons into τ⁺τ^- pairs have not yet been observed, but the LHCb analysis yields the first direct limit on the B_s⁰ branching fraction as well as the world’s best limit on the B⁰ branching fraction.

The first observation of the four-body charmless baryonic decays B_s⁰→ppK⁺K^-, B_s⁰→ppK^±π^∓ and B⁰→ppπ⁺π^- was reported as well as the evidence for the B⁰→ppK⁺K^- decay, see images above. These decays are of great interest in the search for further manifestations of CP violation in baryonic B decays, see the first evidence for the violation of the CP symmetry in baryon decays.

All of these results have been obtained owing to the excellent performance of the LHC collider and LHCb detector. The physics opportunities that exist with an upgraded LHCb detector in the High Luminosity LHC era are even more impressive. A phase-1 upgrade of the experiment is currently being prepared and will be installed in the long-shutdown 2, in 2019-2020. The physics case for the so-called LHCb Phase-II upgrade, following the recent submission of an Expression of Interest by the LHCb collaboration, was also presented for the first time in public.

More details can be found in LHCb presentations at La Thuile: 1, 2 and 3.

14 February 2017: First single experiment observation of the decay B_s⁰→ μ⁺μ^-. From discovery to precision measurement. [ Branching fraction B_s⁰→ μ⁺μ^- = (3.0±0.6^+0.3_-0.2)x10^-9 ; B⁰→ μ⁺μ^- 3.4x10^-10 ]

Today, in a seminar at CERN, the LHCb collaboration presented the first observation by a single experiment of the decay B_s⁰→μ⁺μ^-, with a statistical significance of 7.8 standard deviations. The measured branching fraction (3.0±0.6^+0.3_-0.2)x10^-9 is in agreement with the Standard Model (SM) prediction of (3.65±0.23)x10^-9. It is the most precise measurement of this quantity to date.

The full 3 fb^-1 of data collected during Run 1, and 1.4 fb^-1 of data accumulated during Run 2 were used to obtain this result. A special event selection (BDT for experts) was used to classify data into bins with different ratios of B_s⁰→ μ⁺μ^- decays and background contributions. The μ⁺μ^- invariant mass spectrum for the bins with the smallest background contribution (BDT0.5) is shown in the image. The contribution of the B_s⁰→ μ⁺μ^- decay is clearly visible at the B_s⁰ mass and is indicated as the red peaking distribution. The green peaking distribution shows a possible contribution of the B⁰→ μ⁺μ^- decay at lower mass, which is expected in the SM at a rate of about 30 times smaller. The size of this contribution is not found to be significant, and so an upper limit is set for the decay at a value of 3.4x10^-10. The other contributions show the contribution of background processes. They provide very little contamination in the region of the B_s⁰→ μ⁺μ^- signal.

The probability, or branching fraction, of the B_s⁰ meson to decay into two oppositely charged muons is very small in the SM and is well predicted. On the other hand, a large class of theories that extend the SM, such as, for example, supersymmetry, allows significant modifications to this branching fraction and therefore an observation of any significant deviation from the SM prediction would indicate a discovery of new effects. The decay of a B_s⁰ meson into a muon pair has therefore long been regarded as one of the most promising places to search for these new effects. This decay has been searched for more than 30 years by different experiments at different accelerators as shown in the image. The LHCb collaboration obtained the first evidence, with a significance of 3.5 standard deviations, in November 2012 and, together with the CMS collaboration, the first observation, with a significance of 6.2 standard deviations, in May 2015. Previous results already severely constrained the type of SM-extension models that are still allowed, as described, for example, in the 30 March 2012 news. The results announced today isolate even more precisely the parameter region in which these new models can exist, and therefore focuses future experimental searches and theoretical attention. All candidate models of physics beyond the Standard Model will have to demonstrate their compatibility with this important result.

Some new-physics models also allow the possibility of a different B_s⁰ “effective” lifetime from what is predicted in the SM. LHCb also reported today the first measurement of this quantity, and found it to be 2.04±0.44±0.05 ps in agreement with the SM prediction. The left image shows the characteristic exponential decay time distribution of B_s⁰→ μ⁺μ^- decay events.

A typical B_s⁰→ μ⁺μ^- decay candidate event recorded in 2016 is shown below. The two muon tracks from the B_s⁰ decay are seen as a pair of green tracks traversing the whole detector in the left image. The right image shows the zoom around the proton-proton collision point, the origin of many particle tracks. The two muon green tracks originate from the B⁰_s decay point located 17 mm from the proton-proton collision. You can play with images of 3 new B_s⁰→ μ⁺μ^- decay candidates recorded during the 2016 data taking period using the LHCb 3D event display. This web based event display will run on your computer or smartphone without need to load any specialized software.

LHCb looks forward to making even more precise measurements of the B_s⁰ branching ratio, continuing its search for the B⁰→μ⁺μ^- decay, and improving the knowledge of the effective lifetime. Stay tuned for updates from Run 2 data.

Click the images for higher resolution. More details can be found in the LHCb presentation at CERN and in the LHCb publication. Read more in the CERN update for the public both in English and in French, in the Symmetry magazine and in the CERN Courier article. The preliminary numbers and plots presented at the seminar have been replaced with the final ones on March 11.

30 January 2017: Measurement of matter-antimatter differences in beauty baryon decays. Towards an important milestone in particle physics.

The LHCb collaboration has published today in Nature Physics the first evidence for the violation of the CP symmetry in baryon decays with statistical significance of 3.3 standard deviations (σ). CP violation has been observed in K and B meson decays, but not yet in any baryon decay. If the measurement is confirmed with a statistical significance of 5σ using a larger data sample, it will be the first time that an asymmetry in the decay rate of baryon and an anti-baryon is observed. In the quark model of particle physics mesons are composed of a quark and antiquark pair while baryons (anti-baryons) are composed of three quarks (anti-quarks).

In order to obtain this result LHCb physicists made the first observation of Λ_b⁰ beauty baryon decays into four other particles pπ^-π⁺π^- and pπ^-K⁺K^-, as seen in the images above showing accumulation of events at the Λ_b⁰ mass. About 6000 and 1000 decays were found for the two decay modes, respectively. The beauty baryon Λ_b⁰ is composed of udb quarks and its mass is about 6 times the mass of a neutron (udd).

The two decays proceed mainly through these two “Feynman diagrams”. It is important to measure the size and nature of CP violation in these decays in order to determine whether they are consistent with the predictions of the Standard Model of particle physics or, if not, what extensions of the Standard Model would be required to explain them.

Once the signals have been established, the analysis turned to the study of matter-antimatter asymmetries. Before being observed in the LHCb detector, the particles can “resonate” to form intermediate particles. For example, in the Λ_b⁰ → pπ^-π⁺π^- decay prominent signals of Δ(1232)⁰ particle decaying to a proton and a π^- meson, and rho(770)⁰ meson decaying to a pair of π⁺ and π^- are found. The decay products of Δ and ρ particles define two planes with a relative angle Φ, as seen in the left image below. The measured asymmetries presented in the right image show local differences and, at some values of the angle Φ, they become as large as 20%. The statistical significance of these asymmetries differing from zero is 3.3σ, leading to the first evidence for CP violation in baryon decays. In the past, analogous large effects were also seen in three-body charmless B decays. The analysis of Λ_b⁰ → pπ^-K⁺K^- decays has lower sensitivity due to a smaller number of observed events, and has not revealed evidence for non-zero asymmetries so far.

The full 3 fb^-1 run 1 data sample was used to obtain this result. The number of beauty particle decays recorded by LHCb in run 2 is already larger than that used in this analysis. Therefore in the near future we will know if the evidence reported here will grow towards a 5σ observation.

Read more in the Nature Physics publication, in the CERN update for the public both in English and French, in the CERN Courier article, also in the Symmetry magazine.

5 December 2016: End of 2016 data taking period.

The 2016 data taking period ended this morning. This was a very successful year, both because of the excellent performance of the accelerator and of the LHCb experiment itself.

The left and central images below show the growth in integrated luminosity during different years of LHC operation. Integrated luminosity is a measure of the number of proton-proton collisions produced by the accelerator. The increase of integrated luminosity with time in 2016 was similar to that in 2012; the period of data taking was, however, shorter. This excellent performance is mainly due to the higher efficiency of the accelerator this year. LHCb has recorded 1.67 fb^-1 of proton-proton collisions this year, 5 times more than in 2015, a year in which LHC collider was setting-up its operation at the record energy of 13 TeV. The total integrated luminosity recorded during run 2 reached now the target of 2 fb^-1 compared to 3 fb^-1 collected in run 1.

The number of collected events per unit time for a given physics process, N, is proportional to the instantaneous luminosity, L, which is a measure of the brightness of the colliding beams, and to the characteristic property of interaction, cross-section, σ; N=σL. The cross-section for b- and b-quark production at 13 TeV proton-proton collision is about twice that of run 1 (see 5 August 2016 news, item (1)). Since the integrated luminosity is merely the instantaneous luminosity added up over the time the accelerator is operating, the 2 fb^-1 Run 2 data sample contains a larger number of beauty particle decays than the 3 fb ^-1 Run 1 data sample. The increase of number of recorded charmed particles per unit of integrated luminosity was even higher, by a factor of 5, both due to the higher cross-section at 13 TeV and to improvements in the event selection (so-called “trigger”) during data taking. This is excellent news for future LHCb physics analyses. The right image above shows a typical proton-proton collision event.

The proton collisions with lead ions took place during last three weeks of data taking period. The left image above shows the increase of integrated luminosity as a fuction of time depending in which LHC ring protons and lead (Pb) ions are located; the first one (p or Pb) indicates which beam is pointed towards the one-sided LHCb detector. In total 31 nb^-1 were recorded. The right image above shows a typical high multiplicity proton-lead-ion collision event, the left one below shows a zoom around the collision vertex and the right one below shows the event as seen from above. Particles identified as pions (orange), kaons (red), protons (magenta), electrons (blue) or muons (green) are shown in different colours. In addition to collected p-lead collisions, LHCb collected data from proton-helium interactions at the same time. This was achieving by injecting helium gas as a target directly in the region of the LHCb Vertex Locator (VELO), and recording those occasions when the circulating protons hit these near-stationary helium nuclei. Such a mode of operation is called "fixed target" data taking and at the LHC is unique to LHCb.

A traditional end of the year shutdown period starts now. It is used for maintenance and improvements of the LHC and its detectors. This time it will be longer due to major installation work carried-out in one the LHC detectors. Therefore the name of “Extended Year End Technical Stop”, EYETS, has been given to this shutdown period. LHCb plans for this period include some relatively minor maintenance work on the detector, together with more major interventions on the access lift and the crane in the underground cavern. It is planned that protons will start to circulate again in the LHC at the beginning of May and that the first proton-proton collisions for physics will take place in mid of June.

Click the images for higher resolution.

1 December 2016: New results presented at the CKM workshop.

The LHCb collaboration presented several new results at the 9^th International Workshop on the CKM Unitarity Triangle (CKM2016) taking place this week at the Tata Institute of Fundamental Research in Mumbai, India. A few selected items are listed below.

(1) A surprising first observation of a decay of the charm beauty meson. LHCb searched for the decay of the B_c⁺ meson into charm D⁰ and strange K⁺ mesons and surprisingly found a signal. The B_c⁺ is the only meson consisting of two heavy quarks of different flavour, namely a b and a c quark. Unlike other B mesons, the b quark decay accounts only for a small fraction of B_c⁺ decays, with the c quark decay accounting for the majority of them. The image shows a clear enhancement around the B_c⁺ mass in the D⁰ and K meson invariant mass spectrum. The contribution of the B_c⁺→D⁰K⁺ decay, shown as the red Gaussian distribution, is observed with a statistical significance of 5.1σ. The result is surprising because the measured decay rate is high compared to theoretical predictions, and it was not yet thought that there were sufficient data collected to see a signal. It will be a nice challenge for theoretical physicists to understand the origin of this unexpected result. For experts: The so-called “tree-amplitude” decay B_c⁺→D⁰π⁺ is not yet observed. The B_c⁺→D⁰K⁺ decay proceeds predominantly through penguin and annihilation decay diagrams. This is the first observation of a B_c⁺ decay dependent on such processes.

(2) CP violation and γ angle measurements in B_s→D_sK decays. The image shows the D_sK invariant mass distribution for different charge combinations of D_s and K mesons. The B_s meson contribution is shown as the dashed red line while the other shaded areas represent different background components. A detailed analysis allowed LHCb physicists to extract a value of the Cabibbo-Kobayashi-Maskawa (CKM) angle γ (see introduction in the 5 October 2012 news). The B_s→D_sK decay proceeds via so-called “tree-level” diagrams, in which contributions of new physics are not expected. Together with other “tree-level” γ angle measurements (see item (1) in the 13 March 2016 news) it provides a powerful consistency check of the Standard Model and establishes reference measurements which could then be compared with the γ angle determination from other processes in which signs of new physics effects could show up.

(3) First observation of CP violation in B_s→K⁺K^- decays. The measurement of CP violation in the charmless two body decays B⁰→π⁺π^- and B_s→K⁺K^- was presented. The word “charmless” indicates the fact that there are no particles containing a charm quark in the decay products. CP violation in B_s→K⁺K^- decays has been observed for the first time. The CP-violating asymmetries were measured as a function of the beauty meson decay time, as shown in the image. As apparent in the figure, the characteristic fast B_s meson oscillation pattern is observed.

(4) Study of the γ angle from B^-→D⁰K^*- decays. The potential for the measurement of the γ angle and of various CP violation measurements in B^-→D⁰K^*- meson decays was presented. In this measurement the D⁰ meson decays into various pairs of charged π and K mesons. B^-→D⁰K^- decays have been extensively analysed in the past but the B^-→D⁰K^*- mode is studied at LHCb for the first time. The analysis has demonstrated that high purity is possible in this decay mode at LHCb owing to the absence of any significant physics backgrounds, as shown in the image. While in items (1), (2) and (3) the analyses used the full 3fb^-1 LHC run 1 data sample, this analysis profited from an additional 1fb^-1 collected during run 2 data taking period. The significant increase in signal yield per unit of integrated luminosity in run 2 (see 5 August 2016 news, item (1)) bodes well for improvements in the knowledge of γ in future analyses at LHCb.

See more details in the LHCb CKM workshop presentations linked from the section titles in italic and soon in the LHCb papers and conference notes. Click on the images for higher resolution.

28 September 2016: Charming time asymmetries.

The LHCb collaboration presented many new interesting results at the VIII^th International Workshop on Charm Physics (Charm 2016) which took place in Bologna. In particular, precise measurements on the difference between the lifetime of D⁰ mesons, composed of a cu quark pair, and their partner D⁰ mesons, with opposite quark content cu (when decaying either to a pair of pions or a pair of kaons) were reported, in order to search for CP-violating effects in the Charm sector.

The phenomenon of CP violation, that is related to the difference between properties of matter and antimatter, is still unobserved in the charm-quark sector. As charm mesons are composed of up-type quarks only, this uniqueness makes the study of their properties particularly relevant. Such properties might be sensitive to effects beyond those predicted by the Standard Model. According to the Standard Model, CP-violating asymmetries in the charm sector are expected to be very small, below the 10^-3 level for this measurement. Remarkably, the LHCb experiment is now approaching a level of precision where such small effects could be observed.

Charm mesons are produced either directly in the proton-proton collisions or in the decays of heavier beauty particles. Only the first category was used in this analysis. The difference of the D⁰ and D⁰ meson decay rates into K⁺K^- and π⁺π^- pairs was measured as a function of their decay time t. Two distinct measurements were done, which make use of the same data sample but with different experimental approaches. For the K⁺K^-, the asymmetry of the rates is shown in the images. The variation of these quantities depend on the so-called parameter A_Γ. The numerical results for A_Γ can be found in the LHCb presentation at the workshop as well as in the two conference notes (09) and (10). These are the most precise measurements of CP violation ever made in the Charm sector, and are consistent with no CP violation with a precision of a few parts in 10⁴. Many checks have been done in order to verify the good accuracy of the measurements to such an impressive level of precision. As an example, results obtained with two different orientations of the LHCb magnetic field, "up" and "down", are compared in the left image above for the data taken in 2012. Note that LHCb presented also other interesting measurements of CP violation at Charm workshop, as well as at the ICHEP conference before, see item (2) of the 5 September 2016 news. Read also the CERN update for the public in English and French.

19 August 2016: Excellent performance of LHC and LHCb.

The LHC collider and the LHCb detector continue to work very well. LHCb has collected already 1 fb^-1 (integrated luminosity) of data this year, three times more than in 2015. Given that 7 more weeks of proton-proton collisions remain in this year's schedule, it can be hoped that the final data set will be significantly larger. Taking into account that the beauty-particle production rate at the higher collision energy of run 2 is more than twice that of run 1 (see item (1) of ICHEP 2016 news), the total number of beauty-particle decays collected during run 2 is likely to be already higher than the total of run 1 by the end of this year. To reach this achievement LHCb has profited from the improvements to the data acquisition. In addition, the data accumulated in 2016 benefits from the revolutionary design of the new LHCb trigger.

The proton-proton collision period will end at November 1^st and then will be followed by a LHC machine maintenance period (technical stop) and three weeks of proton collisions with lead ions. The image shows the integrated luminosity progress during the different years of data taking. Follow the progress of data taking by clicking at the links to live information at the top of this page, “LHC and LHCb Status Displays”, “LHCb Event Display”; the frequently updated image reports the LHCb delivered and recorded luminosity.

5 August 2016: New results presented at the traditional ICHEP conference.

The LHCb collaboration presented this week new results at the 38^th International Conference on High Energy Physics, ICHEP, which is taking place place at Chicago. A few selected items are listed below.

(1) Measurement of the b-quark production cross-section in pp collisions at 7 and 13 TeV. The probability of b- and b-quark production (cross-section) in proton-proton collisions can be calculated in the framework of the theory of strong interactions, quantum chromodynamics (QCD). Sizeable uncertainties exist in the absolute predictions, but they are strongly reduced in the ratio R_13/7 of calculations performed for 13 TeV and 7 TeV. The image shows a comparison of these calculations with the results of the LHCb measurement as a function of the variable, pseudo-rapidity η related to the b-hadron production angle θ measured with respect to the direction of incoming proton beams, η=-ln(tan(θ/2)). This image is different from that presented at the conference, as described in the erratum to the corresponding paper. The ratio R_13/7 is 2.00 ± 0.02 ± 0.26 and is in agreement with theoretical expectations over the whole η range under study.

(2) Most precise CP violation measurement of a single decay of charmed particles. The measurements of D⁰ mesons decays into pairs of K⁺ and K^- mesons was reported. This measurement is interesting since charmed D-meson decays are suitable for probing CP violation in the up-type quark sector. CP violation is related to the difference between the properties of matter and antimatter. It is measured here as the asymmetry parameter A_CP, which differs from zero if and only if the probability of the D⁰ meson to decay into a K⁺K^- pair differs from that of the D⁰ meson. Recent studies of CP violation in weak decays of D mesons show a good consistency with the hypothesis of CP symmetry so far, in agreement with the expectation of the Standard Model, which predicts very small violation in the charmed system. This is to be contrasted with K and B meson decays, where CP violation is well established, again in broad agreement with the predictions of the Standard Model. By combining the results presented at the conference with previously reported the most precise determination of CP violation in the D⁰ decays into K⁺ and K^- as well as into π⁺ and π^- meson pairs from a single experiment have been obtained. The image summarises the current situation. The LHCb results presented at Chicago are shown as a dashed green ellipse. The D⁰ and D⁰ decays were identified using a so-called “pion tagged” technique. The dashed blue ellipse shows previous results which used a “muon tagged” technique and the red one represents the combination of both measurements. The black ellipse shows a world combination made by the Heavy Flavor Averaging Group, HFAG, which includes the results of other experiments as well as older LHCb results. As apparent from the image there is no evidence for CP violation yet.

(3) Rarest fully hadronic B decay mode ever observed. The B⁰-meson decay into a K⁺K^- meson pair is observed for the first time ever with a significance of more than 5 standard deviations. This decay was searched for in the past for a long time by other experiments. The left image below shows a contribution of this decay to the the K⁺K^- invariant mass spectrum. It occurs at a rate of less than 1 time in 10 million B⁰ decays. This result will help to refine the QCD calculations of the dynamics governing the decays of heavy-flavoured hadrons. The understanding of this dynamics is a fundamental ingredient in the search for new particles and interactions beyond those included in the Standard Model.

(4) First attempt to probe anomalous photon polarisation in B_s to φγ decays. The b-quark to s-quark transition accompanied by a photon γ is considered as a very interesting process in which signs of new physics could show up. New physics models often predict a different value of photon polarisation than that predicted by the Standard Model. LHCb physicists already succeeded to observe a non-vanishing value of the γ polarisation for the first time in B⁺→K⁺π^-π⁺ γ decays (see 28 February 2014 news for introduction and the measurement details) and have probed its nature in B⁰→K^*e⁺e^- decays. This week the collaboration reported the first measurement of the photon polarisation in B_s to φγ decays. The right image above shows the φγ invariant mass spectrum with a clear accumulation of events at the B_s mass. The lifetimes of the mesons in this sample have been analysed and from this study information has been extracted on the photon polarisation. The result is consistent with the Standard Model prediction within 2 standard deviations.

(5) Measurement of time-dependent CP violation in B⁰ to D⁺D^- decays. The Standard Model angle β of the unitarity triangle is usually measured in the decays of the B⁰ and B⁰ mesons to J/ψ and K_s⁰ mesons, see 3 March 2015 news. The measurement of β in the decay of the B⁰ meson into D⁺D^- mesons is relevant as it enables higher order Standard Model and new physics contributions to be probed and constrained. LHCb reported the measurement of the, so called, C and S observables. The results are consistent with the Standard Model expectation, which in the absence of these additional contributions, are S of around -0.75(-sin2β) and C = 0. The images below show the comparison of the LHCb results with previous measurements by the BaBar and Belle collaborations.

See more details in the LHCb ICHEP presentations linked from the section titles in italic and in the LHCb papers: (1), (2), (3), (4) and (5). Click the images for higher resolution.

28 June 2016: Observation of four exotic-like particles.

Today the LHCb collaboration has submitted for publication two papers (one brief, and the other one containing full details) reporting the observation of four "exotic" particles decaying into a J/ψ and a φ meson, only one of which was well established before. These results were based on a result of a detailed analysis of charged B⁺ meson decays into J/ψ, φ and K⁺ mesons using the full run 1 data sample. The data could not be described by a model that contains only ordinary particles, i.e. composite hadrons built of either a quark and an anti-quark, or three quarks. Each of the four particles is observed with a significance exceeding five standard deviations. The sophisticated analysis of the angular distribution of B meson decay products observed with the LHCb detector (a so-called multidimensional full amplitude analysis) allowed LHCb to determine properties (quantum numbers) of the particles with high precision. The images below show the reconstruction of the contributions (in the J/ψφ invariant mass distribution) marked as peaking structures with different colours. The right image shows the spectrum for the selected events with the φK invariant mass to be above 1950 MeV. The properties of these structures are consistent with their interpretation as four-quark particles, which are considered as "exotic", (hence the “exotic-like” name in the title), although the details of the four quark ccss binding mechanism is still under discussion. The binding mechanism could involve tightly bound tetraquarks or strange charm charged meson pairs D_s D_s^* bouncing off each other and rearranging their quark content to emerge as a J/ψφ system (called a “cusp” by experts; note that J/ψφ and D_sD_s^* systems have the same quark content). The high statistics of the LHCb data set and the sophisticated techniques exploited in the analysis will help to shed further light into the production mechanisms of these particles. More information can be found in the first paper, with further details given in the second.

The interest in these four states is also that they are the only known exotic candidates which do not contain u and d quarks, which are the lightest quarks and those which human beings and the matter around us are made of. As such, they may be more tightly bound than other exotic particles.

The observation of the X(4274), X(4500) and X(4700) particles was announced for the first time today. Evidence for the X(4140) near-threshold J/ψφ mass peak at a level of 3.8 standard deviations was first announced by the CDF collaboration and later confirmed by the CMS and D0 collaborations. Searches for this particle by the Belle and BaBar collaborations gave negative results. The LHCb analysis yields a clear observation of the X(4140), and indicates a particle with similar mass but larger width to the earlier measurements from CDF, CMS and D0. It is important to emphasise that simple `bump-hunting' in the mass spectra is not sufficient to learn about the nature of such complicated hadronic structures. Rather, a multidimensional full amplitude analysis, as described in the two papers submitted today, is crucial for data interpretation, and has allowed LHCb to characterise fully the particles, and to determine their quantum numbers.

These results have been recently presented by LHCb physicists at the Meson 2016 workshop, and at the Rencontres de Blois, FPCP, LHCP and BEACH conferences. The LHCb collaboration has made several other important contributions to the investigation of exotic particles. In February 2013 the quantum numbers of X(3872) were determined. In April 2014 the collaboration published results of measurements which demonstrated that the Z(4430)⁺ particle is composed of four quarks (ccdu). In July 2015 the first observation of two pentaquark particles, i.e. hadrons composed of five quarks, was announced.

Read more in the CERN Bulletin article in English and French, in the CERN Courier article and also in the Symmetry article.

25 May 2016: The VELO team is the fastest.

Last week the LHCb Running Team, VELOcity, won the first place in its category (senior) during the traditional CERN Relay Race.

Contrary to statements of other competitors, based on the team's name, the LHCb team was not using bicycles to win the race. The VELO is the name of the LHCb VErtex LOcator, the precise silicon detector located around the proton-proton collision point. The detector has been “running” successfully since the LHC start up. Its winning formula of precision measurements and proximity to the beam line allows it to locate the point (vertex) precisely where the beauty particles decay, as seen in many images on this page. Today’s race has shown that not only the VELO detector is on track to chase down more physics, but the VELO team is the fastest going.

click the image to see better the winner's smiling faces

9 May 2016: The most precise measurement of the a^s_sl asymmetry. [ a^s_sl = (0.39±0.26±0.20)% ]

Last week at the 16^th International Conference on B-Physics at Frontier Machines, "Beauty 2016”, Marseille, France, the LHCb collaboration presented the updated result of a measurement of the semileptonic asymmetry, a^s_sl, related to a difference between a probability of a beauty meson, B⁰_s, to oscillate into its antimatter partner, B⁰_s, and a probability of the reverse process. (An introduction to beauty and charm oscillations can be found in the 7 November 2012 news item.) Any difference in this probability would be a manifestation of CP-violation, which is the difference between the properties of matter and anti-matter. The label "s" indicates decays of B⁰_s mesons composed of anti-beauty b and s quarks, while "sl" (semileptonic) indicates that leptons, in this case muons, are present among decay products. The full run 1 data sample of 3 fb^-1 was used to obtain this update of the 1 fb^-1 2012 measurement. The LHCb result is the most precise measurement of a^s_sl to date and is consistent with the value predicted in the framework of the Standard Model. For this particular quantity the amount of CP-violation is expected to be tiny and hence the predicted value of a^s_sl in the Standard Model is very small. Therefore the possible contribution of as yet undiscovered effects, which help to drive the B⁰_s - B⁰_s oscillations, could lead to significant changes in a^s_sl. The precise LHCb result allows constraints to be placed on the properties of these possible new effects, and points the way for future theoretical and experimental studies.

The image shows the overview of the most precise measurements of a^s_sl and a^d_sl. The a^d_sl results were obtained from analogous measurements of B⁰-B⁰ oscillations. The new LHCb result is shown, as well as the LHCb a^d_sl 2014 result. The horizontal and vertical bands indicate the naive averages of pure a^s_sl and a^d_sl measurements obtained by different experiments. These averages are consistent with the small values predicted by the Standard Model and show no evidence for new physics effects.

The yellow ellipse shows a result from a measurement of the D0 experiment at the Tevatron which is related but not uniquely determined by a linear combination of a^s_sl and a^d_sl. This result could indicate the presence of a new physics contribution, see the 7 July 2012 news for details. However, this measurement is not in good agreement with the results for the individual measurements of a^s_sl and a^d_sl.

Read more in the LHCb presentation at Beauty 2016, in the LHCb paper and in the CERN Courier article

23 April 2016: Run 2 Year 2 physics commissioning with stable beams.

This night the LHC collider has reached the “Stable Beam” state for the first time this year following the traditional LHC winter shut-down and the collider restart commissioning procedure. In “Stable Beam” conditions the experiments can switch on safely their sensitive sub-detectors. Therefore LHCb physicists were able to activate the tracking detectors as well as the Vertex Locator VELO and to observe particle tracks for the first time this year. The left image shows the LHC main information screen. The right image shows a typical event fully reconstructed during data taking. Particles identified as pions, kaon, etc. are shown in different colours.

Thanks to the joint effort between the SMB, EN and TE CERN departments in collaboration with LHCb, this year data taking is controlled from the new LHCb Control Centre presented in the left image below. The right image is a photograph of a meeting yesterday morning between LHCb physicists in preparation for today's data taking.

Click the images for higher resolution.

The proton intensity in the first “Stable Beam” run was very small with only 3 proton bunches per beam. LHCb can nevertheless use the collected data for physics commissioning with low-intensity stable beams. It is planned that the intensity will be increased during the following runs this weekend. During the coming week the LHC commissioning will continue and the data taking will resume at the beginning of May with increased proton-proton collision intensity. During the first period of operation LHCb will profit from its revolutionary improvement of data acquisition and analysis developed a year ago. The calibration (precise determination of the relationship between the detector response and physical quantity being measured) and alignment process (determination of the relative geometrical locations of the different sub-detectors with respect to each other) now takes place automatically in the computer farm during data taking and the recorded data are immediately available for the physics analysis. Hence it will be possible to begin analysing the new data for physics measurements very rapidly.

Many exciting results have been reported on this web page. They are based on data collected during the three-year Run 1 and the first year of Run 2. In this, the second year of Run 2, it is expected that LHCb will collect substantially more data than in 2015. This larger sample will enable LHCb to obtain even more precise, interesting and, hopefully, surprising results, see video. As before follow LHCb data taking by watching live event display as well as live LHC and LHCb status pages.

11 April 2016: Theatre of Dreams: LHCb looks to future

80 physicists gathered in Manchester on April 6^th and 7^th to discuss the future of the LHCb experiment. The LHCb collaboration is currently constructing a significant upgrade to its experiment which will be installed in 2019/2020. The workshop in Manchester, entitled “Theatre of Dreams: Beyond the LHCb Phase I Upgrade”, explored the longer term future of the experiment in the second-half of the coming decade, and thereafter. The image below shows the Workshop participants at the University of Manchester museum.

In the mid-2020s the LHC will be upgraded for higher luminosity operation. At this time the ATLAS and CMS experiments plan to undertake major phase II upgrades of their experiment. These works will necessitate a long-shutdown of at least 2.5 years duration. The meeting discussed enhancements to the LHCb experiment, dubbed a Phase Ib upgrade, that could be installed at this time. Although relatively modest, these improvements could bring significant physics benefits to the experiment. These include an addition to the particle Identification system using an innovative time of flight system based on Cherenkov light; placing detector chambers along the sides of the LHCb dipole to extend the physics reach by reconstructing lower momentum particles; and replacing the inner region of the electromagnetic calorimeter with new technology, thus extending the experiment’s measurement programme with photons, neutral pions and electrons.

In the second half of the 2020s, the LHCb upgraded experiment that is currently under construction will reach the end of its planned programme. At this time a Phase II upgrade of the experiment could be foreseen. The goal would be to collect an integrated luminosity of at least 300 fb^-1, with an instantaneous luminosity a factor ten above the upgrade that will operate in the 2020s. Promising high luminosity scenarios for LHCb from the LHC machine perspective were shown which would potentially allow this goal to be reached, and each of the elements of the LHCb experiment presented their first thoughts on how these goals might be achieved. The experimental physics programme, the theory perspectives of heavy flavour physics, and the anticipated reach of Belle II and the other LHC experiments were also considered.

Many promising ideas were presented at the workshop and these will be followed up in the forthcoming months to identify the requirements and the RD programmes that will be needed to bring these concepts to reality.

The meeting was sponsored by STFC, Institute of Physics and Institute of Particle Physics Phenomenology and the University of Manchester.

13 March 2016: World's most precise measurements
and search for the X(5568) tetraquark candidate. [ γ=(70.9^+7.1_-8.5)° ; Δm_d=(505.0±2.1±1.0)ns^-1 ; X(5568) not confirmed ]

The LHCb Collaboration has presented today at the Rencontres de Moriond EW new important results.

(1) CKM γ angle measurements. The parameters that describe the difference in behaviour between matter and antimatter, known as CP violation, are constrained in the so called CKM, or unitarity, triangle. The angles of this triangle are denoted α, β and γ, and among these, γ is the least precisely known. A detailed introduction to the measurement of γ can be found in the 5 October 2012 news and in this CERN Courier article. The γ value of (70.9^+7.1_-8.5)° presented today was obtained from the combination of many different LHCb measurements, notably including some new results unveiled today for the first time, and is the most precise determination of γ from a single experiment. One of the new analyses presented today uses decays of charged B mesons into charmed D mesons and pions π or kaons K. Then the D mesons decay in turn into various combinations of π’s and K’s. The image displays the different rates of positive and negative B mesons, clearly indicating different properties of matter and antimatter.

(2) Determination of the B⁰ oscillation frequency. A fascinating feature of quantum mechanics, in which the B⁰_s, B⁰ and D⁰ particles turn into their antimatter partners, has been discussed already few times at this page, see 15 March 2011, 7 November 2012 and 3 March 2013 news. This feature is called oscillation or mixing. LHCb physicists presented today the most precise single measurement of the parameter which sets the B⁰ meson oscillation frequency to be Δm_d=(505.0±2.1±1.0) ns^-1. The full run1 data sample of semileptonic B⁰ decays with charged D or D^* mesons was used in this analysis. The image shows the characteristic oscillation pattern of B⁰ mesons.

(3) Non-confirmation of the X(5568) tetraquark candidate. Two weeks ago the D0 Collaboration at Fermilab reported the observation of a narrow structure, X(5568), in the invariant mass of the B_s⁰ meson and a charged pion π, see Fig. 3 in the D0 publication, and interpreted it as a tetraquark candidate composed of four different quarks (b, s, u and d). An introduction to four-quark systems, or tetraquarks, can be found in the 9 April 2014 news.

The LHCb Collaboration reported today a result of a similar analysis using a sample of B_s⁰ mesons 20 times higher than that used by the D0 Collaboration. The B_s⁰π invariant mass spectrum is shown in the figure using the B_s⁰ mesons decaying into J/ψ and φ mesons or into D_s and π mesons. No structure is seen in the region around the mass of 5568 MeV (indicated by the arrow). Hence the LHCb analysis does not confirm the D0 result. Read more in the LHCb conference note, in the LHCb paper, in the LHCb CERN seminar and also in the LHCb presentation at the Rencontres de Moriond QCD.

11 December 2015: Awards of the LHCb Kaggle competition.

The world’s community of machine learning data scientists organizes “Kaggle” competitions to solve the most difficult and interesting challenges in different fields. This year, for the first time, a competition has been proposed by the LHCb experiment. The aim of the exercise was to establish the best way of looking for a phenomenon that is not (yet?) known to exist – the decay of a tau (τ) lepton into three muons μ’s, denoted τ → μμμ. This decay is forbidden in the Standard Model of particle physics and therefore its observation would indicate a discovery of "new physics", which is now the key goal of the LHC. The awards of this competition were announced today at the Applying (machine) Learning to Experimental Physics workshop organized in the framework of the Twenty-ninth Annual Conference on Neural Information Processing Systems.

Machine (computer) learning is widely used in particle physics in order to select a small number of interesting candidates found in individual collisions, called by physicists “signal events”, among often a large number of uninteresting events called “background”. Some machine learning algorithms are inspired by biological neural networks. They are used to estimate functions that can depend on a large number of input information supplied by physicists, which is different for the signal and background events. The model then “learns” from the simulated signal and background events what the best criteria to separate them are. An output function is then generated indicating if the experimentally measured events look more or less like the signal or background events. Currently the most popular machine learning algorithm used in particle physics is called a “Boosted Decision Tree (BDT)”, see for example, its use in the B_s→ μμ decay observation paper.

673 teams took part in the competition from all over the world. They worked with real data from the LHCb experiment mixed with the simulated data of τ → μμμ decays. The solutions presented by the winning teams were presented at the workshop. The LHCb collaboration have offered also two physics prizes for the most useful solutions for their analysis. The winners will be invited to the Heavy Flavour Data Mining workshop in Zurich in February 2016. The prizes were sponsored by Yandex and Intel.

The competition may allow LHCb physicists to learn some computing tricks from the machine learning community. This community, on the other hand, may possibly have improved their skills by having tackled this very challenging particle physics challenge.

2 December 2015: A mysterious ridge effect.

The LHCb collaboration has submitted today a paper reporting the study of correlations in particle production in proton-lead ion collisions at the LHC. The plots (see below) showing the angular distribution of these correlations exhibit features similar to a “ridge” in a mountain landscape. Therefore physicists name this kind of analysis a study of a “ridge effect”.

Two sets of data were taken: proton-lead and lead-proton, where in the second case the direction of the proton and lead ion beams were reversed. This allowed the LHCb detector, recording the particles only on one side of the interaction point, to make measurements in the case of the proton beam pointing towards the LHCb detector as well as in the opposite case of the lead beam pointing to it.

The idea of the analysis is simple. LHCb physicists plotted differences Δ between production angle of particle pairs both in angle θ along the direction of incoming proton or lead beams, and in angle φ around this direction. The difference in angle θ is plotted as the difference of a related variable, pseudo-rapidity η, where η=-ln(tan(θ/2)). In this way the plot bin sizes closer to the beam direction are expanded.

The Δη-Δφ distributions show three different features. The high peak structure near Δη=Δφ =0 is caused by a clustering of particles, forming a jet-like structure. This peak has a width of about Δη=1 and is truncated in the images in order to allow a better visibility of other features. Since momentum must be conserved, another structure is observed on the opposite side at Δφ=π. The distribution in Δη is, however, broad and is referred to as the “away-side ridge”.

The third structure, the “near-side ridge”, is the most interesting and its study is the main subject of today's paper. It is seen on both sides of the high peak at Δη=Δφ =0 in the images above. Only properties of 3% of events, characterized by the highest “activity”, are contributing to the shown distributions. A very precise Vertex Locator detector (VELO), surrounding the proton-lead interaction region, is used to measure the number of produced particles in the collision and to define the activity of the event. The two plots above show that the near-side ridge is more pronounced in the highest activity events (event class 0-3%) when the lead (Pb) beam is pointing towards LHCb detector (Pb+p) than in the opposite case.

The numbers of particles observed in the LHCb detector are not identical in these two cases. It is higher in the case of lead ions pointing towards the LHCb detector (Pb+p) than in the opposite case (p+Pb). Here LHCb physicists have introduced an original idea to the analysis and also made an important discovery. By comparing events with similar numbers of particles observed in the acceptance of LHCb detector (similar absolute activity), LHCb physicists found out that the sizes of the near-side ridge effect in the direction of the lead (Pb+p) and proton (p+Pb) beams are in fact compatible.

The ridge-like near side effects were studied in heavy ion collision experiments in order to investigate a possible manifestation of a quark-gluon plasma formation, see 10 May 2013 news for an introduction. The observation of the similar near-side ridge correlations in proton-proton collisions by the CMS collaboration and in proton-lead collisions by the LHC experiments came as surprise since the formation of quark-gluon plasma was a priori not expected in these collisions. The interpretation of today’s LHCb results obtained in the forward acceptance is clearly an interesting challenge for theorists.

LHCb, the world's first dedicated b-physics experiment at a hadron collider, has obtained not only excellent heavy flavour results, but in addition the quality of the LHCb detector and its unique forward geometry allows it to obtain also important results in other fields, like electroweak physics or heavy ions physics, as reported today. The LHCb contribution to the heavy-ion physics will increase significantly in the near future since presently, for the first time, the collaboration is taking also lead-lead collision data.

Read more in the LHCb publication.

5 November 2015: 20 birthday candles for LHCb.

In August 1995 a Letter of Intent was submitted for LHCb, the world's first dedicated b-physics experiment at a hadron collider. Today, the LHCb Collaboration has marked the 20^th anniversary of this event with a special celebratory meeting.
Click the cartoon (Adrien Miqueu author) for higher resolution.

The beauty quark b and anti-quark b bound-state Υ was discovered in 1977 at Fermilab. For many years after, the study of beauty hadrons, composed of a b quark or its anti-quark b and other quarks, was dominated by experiments at e⁺e^- colliders, which included DORIS, CESR, VEPP and LEP. So called "fixed-target" experiments at hadron accelerators, at which beauty particles were produced in collisions of accelerated protons with stationary objects were limited in their scope, as the rate of beauty particle production was small compared to production rate of other particles. At hadron colliders, on the other hand, the beauty particle production rate is much higher. The Fermilab collider experiments CDF and D0 at the Tevatron, although not designed specifically for studies of beauty particles, took advantage of this opportunity and started to obtain interesting results in the 1990s. The e⁺e^- collider B factory BaBar and Belle experiments were approved in 1993 and started to take data in 2000.

It was clear that the high proton-proton collision energy at the LHC would give rise to a very high beauty particle production rate. But how to conduct high precision experiments in this very difficult environment? At the LHC workshop in Evian in 1992 three b-physics experiments were proposed:

COBEX proposed to use a forward spectrometer in the proton-proton colliding mode, as the majority of beauty particles are produced around the direction of colliding protons. LHB proposed to use extraction of protons from the collider in a fixed target experiment. GAJET proposed to inject a gas into a collider tube; the gas molecules could then play the role of a fixed target.

In June 1994 the LHC Committee decided not to approve any of the three individual experiments, but requested that the interested parties form one new collaboration to propose a single new experiment based on the collider mode to exploit its large bb cross section with a convincing trigger strategy. Adopting the collider mode was the correct choice since not enough b hadrons would have been produced at the LHC in a fixed target setup, compared with the B-factory and Tevatron experiments, which had performed beyond original expectations.

The Letter of Intent for LHCb was submitted in 1995 and the experiment was approved in 1998. The design of the experiment was re-optimized in 2003, a process in which many important improvements were made. The tracking stations in the magnet were removed, reducing strongly secondary interactions of particles from the beauty particle decays, instead all the first tracking stations were made in silicon technology. The particle interactions in the collider beam tube were reduced by replacing the standard technology with one made of beryllium. Finally, improvements in technology allowed the whole LHCb detector to be read out to the computer farm at a 1MHz rate, thereby improving the beauty particle selection process.

But what could be the name of the experiment and its logo? Alternative name versions were used 20 years ago: “LHC-B”, “LHCB” or “LHCb”. The final name was fixed in 1997 with the cute choice for the LHCb logo. Here the “cb” is transformed in the mirror image into “CP” which is then “violated” by the red bar pointing to “CP violation” as the one of the most important themes of LHCb research.

The ideas imagined 20 years ago have been very successful and allowed excellent physics results to be obtained, as reported on this web page. Beauty hadron distributions can be measured with background levels as low as those obtained at e⁺e^- colliders and they are collected at much higher rate. In addition the production of all beauty mesons and baryons is observed at LHCb contrary to B-factory experiments, which are limited to studing light beauty meson decays only.

Read more about the history of LHCb, its physics results and prospects for future in the LHCb20-fest presentations. By clicking on the detector photos at the LHCb20-fest web page you can learn more about the LHCb detector. By clicking on the physics result images at the same web page you can learn more about significant LHCb results.

14 October 2015: Revolutionary improvement of data acquisition and analysis. The first integrated data-taking and analysis in a High Energy Physics experiment.

The procedure of data-taking and analysis at hadron colliders is performed in two steps. In the first one, called by physicists “online”, the data are recorded by the detector, read-out by fast electronics and computers, and finally a selected fraction of events is stored on disks and magnetic tapes. The stored events are then analyzed later in the so called “offline analysis”. An important part of the offline analysis is the determination of parameters which depend on the data-taking period, for example alignment (determination of the relative geometrical locations of the different sub-detectors with respect to each other) and calibration (precise determination of the relationship between the detector response and physical quantity being measured). The whole data sample needs then to be “reprocessed” by the computers with this new set of parameters. The whole process takes a long time and uses a large amount of human and computer resources.

In order to speed-up and simplify this procedure, the LHCb collaboration has made a revolutionary improvement to the data-taking and analysis process. The calibration and alignment process takes place now automatically online and the stored data are immediately available offline for physics analysis. The new procedure allowed LHCb to present the first LHC run 2 physics results at the EPS-HEP conference just a week after the data-taking period ended. In the following the new procedure is described in more detail.

Current technology does not allow all LHC proton-proton collision data to be stored and analysed. An event selection procedure is therefore necessary following the scientific goals of each experiment. This selection procedure can be made by fast electronics (“hardware trigger” in physicists language) or/and by computers (“software trigger”). At LHCb the fast electronics (hardware trigger) reduces the 30 million per second (30 MHz) LHC proton-proton collision rate (as visible by the LHCb detector) to 1 MHz using the characteristic properties of beauty and charm particle production and decays. The data are then read-out from the whole detector and are transferred at the 1 MHz rate to around 300 electronic cards as the one shown in the left image. The fast calculations performed in these cards allow the data volume to be greatly reduced by removing the content not containing information about the current event. The data from all these cards are then transferred to a predefined computer in the LHCb farm, shown in the right image, situated near the detector 100 m underground. The data transfer speed of the network between the cards and the computer farm is so high that it would be capable to carry the entire mobile phone traffic of a country such as Switzerland. The farm contains about 1800 computer boxes with a total of about 27 000 physical processors, nearly doubling the available processing power with respect to the LHC run 1 period.

The software trigger computer programs, called also “High Level Trigger (HLT)”, run in the computer farm. At the first stage, HLT1, the less interesting events are removed and the data flow rate is reduced to 150 kHz. The selected events are stored in a 5 PB disk “Buffer” (see image). An automatic procedure is then run which aligns around 1700 detector components and calculated about 2000 calibration constants, all within a few minutes. The alignment and calibration parameters are then used in the second stage of the software trigger, HLT2, processing the data stored in the Buffer with the same quality as would be the case in the offline analysis. The additional selection reduces the data flow rate to 12.5 kHz and the reduced data sample is stored for future analysis. The output data are directed into two samples. The larger volume “full” data sample allows the whole offline reconstruction and associated processing to be redone as required. The “turbo” data sample keeps only information necessary to perform physics analysis with the offline quality obtained during the HLT2 processing. This “turbo” sample was used to obtain results presented at the conference just a week after the period of data collection ended, as was mentioned above.

This new approach of making offline-quality information available to the trigger, and performing physics analysis directly on the trigger output data, represents a paradigm shift in data processing for particle physics experiments, and will have significant consequences for the future physics programme of LHCb.

Click the images for higher resolution. Read more in the Cern Courier article and in the LHCb seminar at CERN.

26 September 2015: A measurement of a fundamental parameter of the Standard Model. [ sin²θ_W^eff = 0.23142±0.00073(stat)±0.00052(sys)±0.00056(theory) ]

The LHCb collaboration has submitted a paper based on run 1 data which reports the measurement of a fundamental parameter of the Standard Model (SM), the electroweak mixing angle, θ_W. This parameter quantifies the relative strengths of electromagnetism and the weak force. It is, therefore, an important experimental challenge to measure it.

The squared sine function of the electroweak mixing angle sin²θ_W^eff was precisely measured by the experiments at the Large Electron-Positron Collider (LEP) in the 1990s. The electrons and positrons were collided at the energy of about 91 GeV at which the Z boson resonance was formed. The Z bosons than decayed into pairs of leptons (e⁺e^-, μ⁺μ^-, τ⁺,τ^-) or pairs of quarks. By determining the electric charge and direction of these decaying particles a quantity called the "forward-backward asymmetry" was measured, and then used to calculate the value of sin²θ_W^eff. The forward-backward asymmetry is related to how often the produced matter particle travels in a similar ("forward") direction as the incoming matter particle involved in the collision. An important measurement was made also at another electron-positron collider SLAC Linear Collider (SLC) with the detector SLD. The sin²θ_W^eff parameter was determined from the analysis of a so called left-right asymmetry obtained by counting the difference in number of Z bosons produced with two opposite (left and right) spin orientation of colliding electrons using the longitudinally polarized SLC electron beam. The measurement requires no detailed final-state identification. It turned out that the most precise measurements of sin²θ_W^eff, the LEP forward-backward b quark asymmetry (A_FB(b)) and the SLD left-right asymmetry (A_LR), differed by 3.2σ (see a figure from the LEP-SLD electroweak report in which all the details of measurements are explained). This 3.2σ LEP-SLD puzzle has been interpreted by some commentators as an effect driven by new physics. It is a very interesting challenge for measurements at other particle colliders to resolve this puzzle.

Measurements of sin²θ_W^eff have continued at the Tevatron at Fermilab and LHC at CERN. The task is, however, more difficult than at an e⁺e^- collider. The parton (quark and antiquark) distributions inside proton need to be known precisely. At the LHC the Z bosons used in this analysis are produced mainly in the collisions of (valence) quarks with high momentum and (sea) antiquarks with low momentum. The Z bosons then decay into a pair of electrons or muons. The LHCb geometrical acceptance is ideal for this measurement. The incoming quark direction, needed to define the sign of the asymmetry, can be identified correctly 90% of the time.

LHCb has measured the forward-backward asymmetry A_FB in the angular distribution of muons in dimuon final states as a function of the dimuon mass both at 7 and 8 TeV centre-of-mass energies. An example of the angular asymmetry, for data taken at 8 TeV, is shown in the left image as measurement points compared to a (shaded) Standard Model prediction. The LHCb sin²θ_W^eff result is in very good agreement with the other determinations from LEP, SLD, Tevatron and LHC, and is one of the most precise measurements obtained at a hadron collider. The precision of the measurement is not yet sufficient to shed light on the interpretation of the 3.2σ LEP-SLD puzzle as shown in the right image. However, it is very promising and shows that with the additional data, expected in LHC run 2 and beyond, a very interesting determination of this fundamental parameter should be possible.

Read more in the LHCb publication, in the CERN Courier article and in the LHCb seminar at CERN.

27 July 2015: Is a b to a u quark transition modified by a new particle?

The LHCb collaboration published today in Nature Physics a paper based on run 1 data which reports the determination of the parameter |V_ub| describing the transition of a b quark to a u quark. This measurement was made by studying a particular decay of the Λ_b⁰ baryon. Other measurements of |V_ub| by previous experiments had returned two sets of inconsistent results, depending on which method was used to determine the parameter. Theorists had suggested that this discrepancy could be explained by the presence a new particle contributing to the decay process, which affected the result differently, depending on the measurement method. Today's result from LHCb removes the need for this new particle, while the puzzle of why the original sets of measurements do not agree persists.

The Λ_b⁰ baryon is like a proton, but containing a beauty (b) quark in place of one of the up (u) quarks such then when the b quark decays into an u quark it transforms the Λ_b⁰ into a proton. A W boson is emitted in this process and decays in turn into a muon μ and a neutrino ν_μ. The measurement of decays involving a neutrino is very challenging at a proton collider and it was quite a surprise that this measurement could be done. The probability of the Λ_b⁰→pμν_μ decay depends on the SM parameter |V_ub|. The image shows two simultaneous proton-proton collisions inside the LHCb detector shown by the pink ellipses. The Λ_b⁰ baryon is produced in the right hand side collision and travels a distance of about 1 cm until it decays into a proton (orange), a muon (pink) and an invisible neutrino.

The |V_ub| is a parameter of the 3x3 Cabibbo-Kobayashi-Maskawa (CKM) matrix. In the SM the CKM matrix describes the decay of one quark to another by the emission of a W boson. While the SM does not predict the values of the parameters of the CKM matrix, the measurements of these parameters in different processes should be consistent with each other. If they are not, it is a sign of physics beyond the SM. The consistency is visualized with a help of so called “unitarity triangle”. The LHCb |V_ub| result determines a length of the triangle side opposite to the angle β in the left image below, which together with the measurement of the angle γ gives a consistent SM description. On the other hand, the |V_ub| result marked “Inclusive” does not, as will be discussed below.

The Λ_b⁰→pμν_μ decay belongs to a class of so called “exclusive” measurements meaning that a specific (exclusive) decay is used. The Babar and Belle collaborations have in addition measured the decay rate in an “inclusive” way by summing over all possible B meson decays containing the b to u quark transition. Their results from exclusive and inclusive measurements showed a large difference. This could be explained by a new particle, in addition to the W boson, contributing to the quark transition. (For experts: if this new particle had a right-handed coupling, as opposed to the W boson that only interact with left-handed quarks, the inclusive and exclusive results could be made to agree.) In the right figure above, the crossing of the purple and the red band at about -0.2 on the ε_R x-axis shows that the new particle would have to have a strength of about 20% with respect to the W boson for the quark transition. The measurement from LHCb using a Λ_b⁰ decay gives a different dependence of the transition probability from a new particle as marked with the green band. This is due to the different spin of the Λ_b⁰ baryon compared to the B mesons used previously. That the crossing of the purple band with the green band is exactly at zero removes the need for a new particle. However, it still leaves the puzzle as to why the inclusive and exclusive measurements do not agree. Further intensive research, both at the experimental and theoretical level, will continue to try to understand this disagreement.

Click the images for higher resolution. Read more in the LHCb publication and in Nature Physics News Views.

24 July 2015: First LHC run2 physics results. Measurement of J/ψ production cross-sections in pp collisions at 13 TeV. [ σ(prompt Jψ) = 15.35±0.03±0.85 μb; σ(Jψ-from-b-hadron) = 2.36±0.01±0.13 μb; σ( bb) = 518±2±53 μb ]

At the EPS-HEP 2015 conference in Vienna the LHCb collaboration has presented today the first measurement of the probability to produce a J/ψ meson in proton-proton collisions at 13TeV. Using this measurement they also determined the rate at which beauty quarks are produced at this new, higher energy.

The first task of physicists when operating an experiment at a higher energy is to measure the probabilities of well-known processes. These can be compared to theoretical predictions to establish a firm base upon which to build searches for new physics. The probabilities are related to quantities known as “cross-sections”, σ. How to measure these probabilities is explained in the "How bright is the LHC?" news.

The J/ψ was discovered on 11 November 1974. The importance of this discovery is highlighted by the fact that the subsequent, rapid changes in high-energy physics at the time have become collectively known as the "November Revolution". The spokespersons of the experiments who made this discovery, Richter and Ting, were rewarded for their shared discovery with the 1976 Nobel Prize in Physics. The J/ψ is composed of a charm quark c and an anti-charm quark c. At the LHC, collisions containing J/ψ meson decays are used as an excellent tool for detector calibration, as well as for the first cross-section measurements at new energy frontiers.

The production of J/ψ mesons can be described in two stages. In the first stage a cc quark pair is produced and in the second stage the cc pair forms a J/ψ meson. The first stage can be calculated with the theory of strong interactions, QCD. On the other hand, the second stage, after forty years of theoretical and experimental efforts, is still not fully understood. The J/ψ mesons that are produced in this way are called “prompt J/ψ”. J/ψ mesons can also be observed as a product of of the decays of beauty hadron (containing b quarks) and therefore this component is called “J/ψ-from-b-hadron”.

The two components are clearly visible in the left image which shows the J/ψ decay time distribution with respect to the pp collision time. The data are shown as black points with error bars, the solid red line shows the best data interpretation while the prompt J/ψ contribution is shown in cross-hatched blue. The J/ψ-from-b-hadron contribution in black falls exponentially with a time constant characteristic to the lifetime of beauty hadrons. The right image shows an example of the ratio distribution between the rate of J/ψ-from-b-hadron production at 13 TeV and 8 TeV. The cross-section of J/ψ-from-b-hadron decay is used to compute the total beauty quark pair bb cross-section. The expected rise of the beauty particle production rate of about a factor 2 with respect to run 1 at 8 TeV is confirmed by the data. This increase in rate will enable LHCb to obtain even more precise, interesting and, hopefully, surprising results in the LHC run 2 as explained by Barbara, Mika and Patrick.

Read more in the LHCb presentation in Vienna, in the LHCb publication and in the LHCb seminar at CERN.

14 July 2015: Observation of particles composed of five quarks, pentaquark-charmonium states, seen in Λ_b⁰ → J/ψpK^- decays. [ m(P_c⁺(4450)) = 4449.8±1.7±2.5 MeV, Γ = 39±5±19 MeV ] [ m(P_c⁺(4380)) = 4380±8±29 MeV, Γ = 205±18±86 MeV ]

The LHCb collaboration submitted today a paper based on run 1 data which reports the observation of pentaquark-charmonium states decaying into a J/ψ meson and a proton p. In the traditional quark model, the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are called exotic hadrons. In his fundamental 1964 paper, in which he proposed the quark model, Gell-Mann mentioned the possibility of adding a quark-antiquark pair to a minimal meson or baryon quark configuration. It has taken 50 years, however, for measurements to be performed that unambiguously demonstrate the existence of these exotics. In April 2014 the LHCb collaboration published results of measurements which demonstrated that the Z(4430)⁺ particle, first observed by the Belle collaboration, is composed of four quarks (ccdu). Today, the collaboration has announced the observation of a pentaquark, that is a hadron consisting of five quarks.

LHCb physicists have analyzed a sample of about 26 000 Λ_b⁰ → J/ψpK^- decays with only 5% of background contamination. The Λ_b⁰ baryon is like a neutron, but containing a beauty quark in place of one of the down quarks. This decay can proceed by the diagram (a), which involves conventional hadrons and is dominated by Λ^* resonances that decay in turn into a proton p and K^- meson. It can also have exotic pentaquark contributions, shown in diagram (b), that result in resonant structures (called P_c⁺ in today's paper) at 4380 and 4450 MeV in the J/ψp invariant mass spectrum shown in the left image below. The P_c⁺ particles decaying into a J/ψ meson and a proton must have a minimal quark content ccuud, and are therefore called pentaquark-charmonium.

Claimed discoveries of pentaquark states by other experiments in the past have turned out to be spurious. LHCb physicists have therefore performed a thorough analysis to demonstrate that the signals observed in their data cannot be produced by conventional hadrons, and that they have the properties expected from an exotic resonance, that is a short-lived particle lying outside the traditional quark-model.

LHCb physicists first tried to describe the data with conventional hadrons from the traditional quark model, including 14 Λ^* resonances. As this did not give a satisfactory description of the data, they tried to add one P_c⁺ state, and when that was not sufficient they added a second state. The first one has a mass of 4449.8±1.7±2.5 MeV and a width of 39±5±19 MeV, while the second is wider, with a mass of 4380±8±29 MeV and a width of 205±18±86 MeV. The statistical significance of each of these resonances is more than 9 standard deviations. The J/ψp invariant mass spectrum from the data (below, left) is shown as solid (black) diamonds with error bars, while the solid (red) points show the results of the best data interpretation taking into account quantum mechanical description of P_c⁺ states formation in the presence of conventional Λ^* resonances. The blue open circles with the shaded histogram represent the contribution of the P_c⁺(4450) state. The purple filled circles connected by the histogram represent the P_c⁺(4380) state. This wider state is clearly seen in the insert for which a more restricted range of the Kp invariant mass spectrum (above 2 GeV) was required. Each Λ^* component is also shown in different colors.

In the final step of the analysis the LHCb physicists set out to prove that both P_c⁺ structures really possess the properties of a resonant particle, that is a quantum state with well-defined quantum numbers, which is produced, lives for some time, and then decays. The images above prove to experts that this is indeed the case. The central one shows the so called Argand diagram indicating that the P_c⁺(4450) structure seen in the data (black points) represents really the resonant particle production and decay, since it approximately follows a circular path (red circle), as is expected for a resonant particle. The right one shows the so called Dalitz plot in which a distinct horizontal band near 19.5 GeV² in the J/ψp invariant mass squared indicates the resonant P_c⁺(4450) and P_c⁺(4380) contributions. The analysis of angular distributions of the decay products leads to the most probable assignment of the two P_c⁺ quantum numbers J^P to be 3/2^- and 5/2⁺ although two other combinations are possible.

A typical Λ_b⁰ → J/ψpK^- decay is shown above. The Λ_b is produced in the pp collision point, the origin of many particle tracks, and decays at a distance of 3.9 cm into a J/ψ meson, a K meson and a proton p. The J/ψ meson decays in turn into a μ⁺ and μ^- pair. The kaon K and the proton p particles traverse the LHCb detector and are absorbed in the hadronic calorimeter. The two muons (μ) leave the calorimeter system and traverse the whole muon detector system. The event can also be seen in the top view as well as in the front view of the Λ_b decay point region.

A new field of research now opens up. Scientists will try to understand the “internal mechanism” of quark interactions inside pentaquarks. The two possibilities are illustrated in the figure. The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The quarks could be tightly bound, or they could also be loosely bound in meson-baryon molecule, in which color-neutral meson and baryon feel a residual strong force similar to the one that binds nucleons together within nuclei. It is hoped that future measurements by LHCb will help to answer this question.

Click the images for higher resolution. Read more in the LHCb publication in arXive and in PRL, in the CERN Press Release in English and French, in the CERN Courier article, in the Nature news, in the APS Viewpoint, in the Symmetry magazine, in the Quantum Diaries blog and in different media articles. See also the Fermilab video explaining the LHCb observation and the Gell-Mann's comment.

Added in August 2016: see APS Synopsis, "Pentaquark Discovery Confirmed", and two papers showing that the evidence for pentaquarks in the discovery data is model independent and that the analysis of the Λ_b⁰ → J/ψpπ^- decays confirmes observation of the two pentaquark particles.

3 June 2015: First Physics at 13 TeV.

LHC Run 2 started today. LHCb physicists have been eagerly awaiting this moment since 14 February 2013 when the last Run 1 collisions took place. The LHC has reached the “Stable Beams” state for the first time for two years. In these conditions the experiments could switch on safely their sensitive sub-detectors. This allowed LHCb physicists to activate the tracking detectors as well as the Vertex Locator VELO and to observe particle tracks for the first time. The right image shows a typical event fully reconstructed during data taking. Particles identified as pions, kaon, etc. are shown in different colours. The left image shows LHCb physicists in the control room.

LHCb has published many results based on data collected during the first three-year Run 1. But this was only the beginning. Collisions at 13 TeV will double the production rates of beauty hadrons enabling LHCb to obtain even more precise, interesting and, hopefully, surprising results.

Follow LHCb data taking by watching live event display as well as live LHC and LHCb status pages.

Read more in the CERN Press Release.

25 May 2015: An intriguing anomaly. Measurement of the decay B⁰ → D^*+τ^- ν_τ [ Branching fraction ratio B⁰ → D^*+τ^- ν_τ/ B⁰ → D^*+μ^- ν_μ = 0.336±0.027±0.030 ]

Today, at the 13^th Flavor Physics and CP violation conference in Nagoya (Japan), the LHCb collaboration presented the preliminary result from a measurement of the branching fraction ratio R(D^*): B⁰ → D^*+τ^- ν_τ/ B⁰ → D^*+μ^- ν_μ. The result 0.336±0.027±0.030 is larger than the Standard Model (SM) expectation of 0.252±0.003 by 2.1 standard deviation (σ).

In the SM all charged leptons, such as taus (τ) or muons (μ), interact in an identical fashion (or, in physicists’ language, have the same "couplings"). This property is called "lepton universality". However, differences in mass between the leptons must be accounted for, and affect decays involving these particles. The τ lepton is much heavier than the μ lepton and therefore the SM prediction for the ratio R(D^*) is substantially smaller than 1. This ratio is considered to be precisely calculable thanks to the cancellation of uncertainties associated with the B to D^* meson transition.

Any measurement exhibiting a conclusive breakdown of lepton universality, after mass related effects are accounted for, would be a clear sign of new physics. The ratio R(D^*) is particularly interesting since a large class of SM extensions contain new interactions that involve third generation of quarks and leptons, like here a b quark (from a B hadron) and τ^- and ν_τ leptons. In particular, the presence of additional charged Higgs bosons, which are often required in these models, can have a large effect.

The image shows a comparison of different results for R(D^*). Already a previous measurement from the BaBar collaboration was found to be 2.7σ above the SM prediction. Therefore, the particle physics community has been eagerly awaiting new results. The LHCb measurement confirms the behaviour seen by BaBar. A new measurement from the Belle collaboration, (marked "Hadronic tag") also presented at Nagoya, lies closer to the SM, but is also consistent with the BaBar and LHCb measurements. An older, less precise measurement by Belle (marked "Inclusive tag") is compatible with the same picture.

Taken together these results constitute an intriguing anomaly, and one that is sure to provoke much discussion. Future measurements from LHCb with the run-1 data set, and the data sample to be collected in run 2, will allow for the precision on R(D^*) to be improved further. The R(D^*) anomaly takes its place alongside the R_K puzzle (3 June 2014 news) in hinting that the SM assumption of lepton universality may be incorrect.

The LHCb R(D^*) measurement is the first measurement of this quantity at a hadron collider and also represents the first measurement of any decay of a B meson into a τ lepton at a hadron collider.

Click the image for higher resolution. Read more in the LHCb presentation at Nagoya, in the LHCb publication, in the Scientific American news, in the Nature news, in the Nature Research Highlights and in the Guardian blog.

21 May 2015: First 13 TeV collisions.

Today, proton beams collided at 13 TeV for the first time inside the LHCb detector. The collisions were part of the ongoing accelerator tests, and are not useful for physics studies. The collected data are, however, useful for refining the synchronization of the readout time of different parts of the calorimeters and muon detectors. The image displays an event taken today.

These collisions were the latest in a sequence of careful steps to prepare the LHC for physics operation at 13 TeV. Proton beams first circulated again after a two year long shutdown on the 5^th April, and collided at the relatively low energy of 450 GeV on the 5^th May. First collisions for physics studies, in so-called “Stable Beams” conditions, are expected in early June.

A new LHCb sub-detector, HeRSCheL, also collected today its first proton-proton collisions data. HeRSCheL, a system of forward shower counters, was installed during the two-year shutdown for distinguishing between processes where the interacting proton in the beam remains undetected passing down the beampipe with no activity recorded in HerScheL and processes in which a signal is observed. It consists of several stations, like the one shown in the video, located around the LHC beam pipe in the accelerator tunnel on both sides of the LHCb detector. It is interesting to note that some of these detectors are over 100m away from LHCb, but still see particles from the same collision.

Click the image for higher resolution. Read more in the CERN news.

13 May 2015: Observing nearly invisible. Observation of the rare B_s⁰→μμ decay from the combined analysis of CMS and LHCb data [ Branching fraction B_s⁰→μμ = (2.8^+0.7_-0.6)x10^-9 ; B⁰→μμ = (3.9^+1.6_-1.4)x10^-10 ]

The CMS and LHCb Collaborations have published today in Nature the first observation of the decay B_s⁰→μμ, with a statistical significance exceeding six standard deviations, and the best measurement of its branching fraction to date. Furthermore, a three standard deviation evidence for the decay B⁰→μμ is also obtained. Both measurements are statistically compatible with Standard Model (SM) predictions and allow stringent constraints to be placed on theories beyond the SM. This is one of the most important results obtained by the LHC experiments from the run 1 and tests the SM to the ninth decimal place. The excellent performance of the CMS and LHCb detectors and their data analyses was crucial in obtaining this result as well as the outstanding performance of the LHC itself. The two experiments found a total of about 100 B_s⁰ and B⁰ decays into two muons in a sample comprising 10¹² beauty hadrons collected during 2011 and 2012.

The SM of particle physics describes the fundamental particles and their interactions via the strong, electromagnetic, and weak forces. It provides precise predictions for measurable quantities that can be tested experimentally. The SM predicts that the B_s⁰→μμ and B⁰→μμ decays are very rare, with about four of these decays occurring for every billion B_s⁰ mesons produced and one decay for every 10 billion B⁰ mesons. Prior to the start of operation of the LHC, no evidence for either decay mode had been found, despite around 30 years of searching at previous experiments. Upper limits on the branching fractions were an order of magnitude above the SM predictions.

The probabilities, or branching fractions, of the B_s⁰ and B⁰ mesons to decay into two oppositely charged muons are very small in the SM and are well predicted. On the other hand a large class of theories that extend the SM, like supersymmetry, allows significant modifications to these branching fractions and therefore an observation of any significant deviation from the SM prediction would indicate a discovery of new effects. The B_s⁰ and B⁰ meson decays into a muon pair have long been regarded among the most promising class of measurements where these new effects could show up. Previous LHCb results already severely constrained the type of SM-extension models that are still allowed, as described in the 30 March 2012 news. The results announced in today's publication isolate even more precisely the parameter region in which these new models can exist, and therefore focuses future experimental searches and theoretical attention. All candidate models of physics beyond the Standard Model will have to demonstrate their compatibility with this important result.

The two collaborations first released their individual results for B_s⁰ meson decay as described in 24 July 2013 news. While the results were in excellent agreement, both fell just below the five sigma statistical precision historically needed to claim an observation. The combined analysis improves the precision of the results and in the same time easily exceeds the 5 sigma requirement, reaching 6.2 sigma. This is the first time that a combined analysis of sets of data from more than one LHC experiment has been performed.

The B_s⁰ and B_s⁰ mesons are produced in the pp collision point and decay into a muon pair after a distance of the order of 1cm. The left image shows the B_s⁰ meson decay into two muons presented as green tracks traversing the whole detector. A few other B_s⁰→μμ decay images can be found at this web page (1, 2, 3). The right image shows one of the μ⁺μ^- invariant mass spectra released today, in which an excess, attributed to the signal decays, can be clearly seen above the expected background shape.

The LHCb and CMS experiments will resume data taking in June with proton-proton collisions at a centre-of-mass energy of 13 TeV, which will approximately double the production rates for B_s⁰ and B⁰ mesons and lead to further improvements in the precision of these crucial measurements.

Read more in the Nature publication, in the CERN Press Release, in the Nature news, in the CERN news in English and French and in the theorist's comment in CERN Courier.

5 May 2015: First 2015 proton-proton collisions.

Today, for the first time in over two years, protons have collided inside the LHCb detector. Each proton beam had an energy of 450 GeV, which is the value they have when injected from the SPS to the LHC.

Although these collisions are not at the nominal energy of 13 TeV, and therefore not aimed for physics studies, the collected data are useful for precise synchronization of the readout time of different parts of the calorimeters and muon detectors with the time at which different particles originating from the proton-proton collision point traverse them.

The left image shows LHCb physicists in the LHCb control room. The right image displays an event taken today. Click the picture and play with the 3D view of a few recorded events.

In a follow up step, most probably tomorrow, LHCb physicists will inject neon gas into the LHC vacuum tube in order to measure the shape of the proton beam, by seeing where the proton-gas interactions occur. This beam-gas imaging method, used only at LHCb, allows the "luminosity" of the colliding beams to be determined, and is described in the news item of 7 October 2014. The luminosity is a vital component in determining how often different physical processes occur in proton-proton collisions. Measurements of these processes at the record proton-proton collision energy of 13 TeV are the among the first physics goals of LHCb at the restart of data taking in June.

5 April 2015: LHC proton beams are circulating again.

Today, for the first time in two years, both proton beams are circulating again in the LHC. The picture shows the LHC operators steering the two beams. The proton beam already traversed the LHCb detector one month ago and then continued through one quarter of the LHC circumference, see 7 March 2015 news.

The two month period of re-commissioning with beam starts now. The LHC magnets are already “trained” to accept the high current needed to produce the magnetic field necessary to hold the protons in their orbit with a record energy of 6.5 TeV inside the LHC ring. Therefore the prospects for reaching the proton-proton collision energy of 13 TeV at run 2 are very good.

The LHCb detector and its data acquisition system are ready to take data at this highest proton-proton collision energy. LHCb physicists still continue to analyse the run 1 data obtaining exciting results, many of which have been reported on this website. They are looking forward to the first 13 TeV collisions that are expected in June and are convinced that a bright and exciting future lies ahead.

Read more in the CERN Press Release in English and French and see the video from the LHC restart.

20 March 2015: B⁰→K^*μ⁺μ^-: new analysis confirms old puzzle. Angular analysis of the B⁰→K^*μ⁺μ^- decay.

Today at the 50^th Rencontres de Moriond Electroweak, in La Thuile (Italy), the LHCb collaboration presented the result of a first full angular analysis of the B⁰→K^*μμ decay using its full LHC run 1 data sample. A previously published analysis of the experiments 2011 data sample found a deviation with respect to a calculation based on the Standard Model, see 9 August 2013 news. The particle physics community has been eagerly awaiting the results from the full data sample ever since.

The analysis of the B⁰→K^*μμ decay is considered as a very promising way to search for effects of yet undiscovered particles, see the 14 June 2013 news for an introduction. Unfortunately, the analysis of this decay is also complicated; the best sensitivity to the new particles comes from the study of the angular distribution of the muons and the kaon and pion from the K^* meson decay. Physicists from the LHCb experiment have studied different angular observables as functions of the mass of the muon pair. It was one of these observables, "P₅^'", that showed a local deviation with respect to the Standard Model calculation (at a level of 3.7σ in the mass squared of the muon pair region q² from 4.3 to 8.68 Gev²/c⁴). There is particular interest in these observables because their theoretical prediction is much less dependent on a good understanding of the hadronic physics involved in turning a B meson into a K^* meson (so called form-factors). These observables are therefore ideal for searching for the effects of new particles in this decay.

The image shows the distribution of the P₅^’ observable as functions of the mass squared of the muon pair q². The black points show the LHCb results presented for the first time today. The Standard Model predictions are presented as orange boxes. These were taken from calculations described in a recent theoretical paper. The behaviour seen in the 2011 data sample, shown as the blue points for comparison, is confirmed using the full data set. The measurements in the q² region between 4 and 8 GeV²/c⁴ are both 2.9σ from the Standard Model calculation.

The results of the analysis of the B⁰→K^*μμ decay will continue to attract the attention of the particle physics community in the coming years. In the next couple of years, the LHCb collaboration will improve the precision of their analysis with the help of data collected in run 2 of the LHC. It is also anticipated that the theoretical predictions (the orange regions in the image) will improve in precision. Theorists will be busy trying to make sense of this measurement, and seeking for possible associations with other unexpected effects found in similar decays, for example the R_K anomaly (see 3 June 2014 news). Stay tuned for future developments on this page.

Read more in the LHCb presentation at La Thuile, in the LHCb conference note, in the CERN news, in the CERN Courier article, in theoretical physicists' blogs here and here, in the theorist's comment in the CERN Courier and in the LHCb paper.

7 March 2015: The proton beam has traversed the LHCb detector. The LHCb collaboration is ready to take 13 TeV proton-proton collision data.

This weekend the LHC proton beam has traversed the LHCb detector for the first time in over two years. At the end of November 2014 the proton beam already arrived at a stopper placed in the accelerator known as the “TED”, located at the end of the SPS-LHC transfer line about 300m from the LHCb detector, see 23 November 2014 news.

This weekend the TED exercise was repeated and the LHCb Collaboration recorded again muons produced by protons absorbed in the TED. The TED was then opened and the proton beam entered the LHC ring and was first absorbed in another stopper called the “TDI” located inside the LHC ring about 50m from the LHCb detector. A very large number of particles produced during this absorption process traversed the LHCb detector. Later on the TDI was opened, the proton beam traversed the LHCb detector inside its beam pipe, then travelled through one quarter of the LHC circumference and finally arrived to the LHC beam dump area. The left image shows places, called “hits” by physicists, at which muons from the proton beam interaction at the TED traversed different LHCb sub-detectors. Click the picture and play with the 3D view of these events.

Both proton beams are expected to make the full turn of the LHC collider by the end of March and the first proton-proton collisions at the nominal Run 2 energy of 13 TeV are expected by the end of May.

The LHCb collaboration is ready to take high energy proton-proton collision data. The two year Long Shutdown (LS1) period offered an opportunity for prolonged access, and hence an extensive programme of consolidation and maintenance work. In summer 2014 a detailed field measurement of the LHCb dipole magnet was performed followed by the re-installation of the beam pipe, see the left image, through which proton beams will circulate in both directions. One section of the beryllium beam pipe was replaced. The new beam pipe support structure is now much lighter and therefore unwanted interactions with it of particles measured inside the LHCb detector are strongly reduced.

Read more in the CERN news1 and news2, in the 24 January 2014 “underground” news, in the CERN PH department newsletter, in the CERN Bulletin article and in the CERN Courier article.

3 March 2015: Matter-antimatter trigonometry with LHCb. Precise measurement of the unitarity triangle angle β.

Today at Les Rencontres de Physique de la Vallée d'Aoste, La Thuile, Italy, the LHCb collaboration presented an important result in our quest to understand the nature and origin of CP violation, which is a difference in behaviour between matter and antimatter. The result, derived from a careful analysis of the full run 1 data sample, is a measurement of the angle β of the ‘unitarity triangle’. This triangle is a geometrical representation of CP violating and associated parameters in the Standard Model. One side is defined to have unit length, the other two sides and three angles can be measured independently in different decays of beauty hadrons. It is the task of experimental physicists to measure these properties and see if they provide a consistent description of the triangle. Any discrepancy would point to signs of New Physics beyond the Standard Model. LHCb has already performed the world’s most precise measurements of γ, one of the other triangle angles, see 11 September 2014 news, and the mixing frequency of B_s mesons, see 7 November 2012 news, which is an essential ingredient for the determination of the side opposite to the angle γ.

The unitarity triangle is shown in the above left figure, with each experimental input represented by a coloured region. The result for β is given by the hatched blue diagonal band, marked as sin2β_LHCb. The measurement is made from studying the decays of about 41500 B⁰ and B⁰ mesons to J/ψ and K_s⁰ mesons. The B⁰ mesons can decay to J/ψK_s⁰ in two ways. They can decay directly to J/ψK_s⁰ or they can oscillate, see 7 November 2012 news, into their antimatter partners B⁰ which in turn decay also to J/ψK_s⁰. (This possibility of two paths in the decay process is a B physics analogue of the classical quantum mechanics two-slit experiment, which is described in this pedagogical video). The interference between the amplitudes for the two decay paths results in a time-dependent asymmetry between the decay time distributions of B⁰ and B⁰ mesons, as seen in the right image above. The oscillation amplitude measures sin2β, and gives the magnitude of CP violation present in the process. A value of sin2β=0 would indicate no CP violation. LHCb physicists announced today the value of sin2β=0.731±0.035±0.020, which is consistent with the geometrical expectation provided by the measurements of the other parameters of the unitarity triangle, and hence with the predictions of the Standard Model itself.

This result also confirms earlier measurements of the same quantity performed by the e⁺e^- collider B factory experiments BaBar and Belle. These experiments were constructed specifically to measure sin2β. Their results were vital in confirming the broad validity of the Standard Model description of CP violation and led to the award of the 2008 Nobel Prize of Physics to the Japanese theorists M.Kobayashi and T.Maskawa, who had been central in developing this description. The solid blue diagonal band shows the new world combination of the B factory measurements with the new LHCb result.

Although the LHCb result for the angle β is not yet as precise as the combined result coming from the average of the B-factory measurements, it is similar in precision to the J/ψK_s⁰ analyses of the individual experiments. It has been long-known that this measurement is a priori more difficult to perform in a hadron collider, such as the LHC, but the new result presented at La Thuile is an emphatic statement that LHCb can significantly contribute to our knowledge of this fundamental parameter. A still more precise result will be achievable with the data to be collected in run 2, which will allow for more stringent tests of the Standard Model.

Read more in the LHCb presentations at La Thuile, in the LHCb publication and in the CERN Courier article.

23 November 2014: The proton beam knocks at the LHC door. The LHCb collaboration took proton interaction data this weekend.

The proton beam knocked at the LHC's very solid door this weekend and found it still closed, but nonetheless managed to provide the LHCb collaboration with very interesting data. The CERN accelerator system (see video) is now fully operational, except for the LHC collider itself. This past weekend, CERN accelerator system operators tested the two transfer lines between the SPS and LHC. One of these lines ends with a so-called beam stopper known as the "TED", located at the end of the line about 300m from the LHCb detector. The TED is currently closed, and so absorbed the proton beam before it could enter the LHC. However many muons were produced during the absorption process, and these muons passed through the TED and traversed the LHCb detector.

This “beam dump” experiment therefore created an excellent opportunity for LHCb physicists and engineers to commission the LHCb detector and data acquisition system. The collected data are also useful for detector studies and alignment purposes (i.e. determining the relative geometrical locations of the different sub-detectors with respect to each other).

The image shows the shift leader, run coordinator, spokesperson and sub-detector experts in front of the LHCb data acquisition computer screens.

LHCb took its last collision data on 14^th February 2013. The two year Long Shutdown 1 (LS1) period that followed has been used for an extensive program of consolidation and maintenance (see 24 January 2014 “underground” news). Collisions are expected to resume again in Spring 2015.

Click the images for higher resolution and read about the LHC side of the story here.

20 November 2014: LHCb at the Open Data portal.

The LHC experiments have today released physics data at the OpenData.cern.ch portal.

The LHCb collaboration has contributed with its International Masterclasses exercise, in which the lifetime of the charm particle called the D⁰ meson may be measured using real proton-proton collision data recorded by the LHCb experiment during the 2011 data taking period. Details of the LHCb exercise can be found at the LHCb masterclasses web pages.

Read more in the CERN press release in English and French.

19 November 2014: First observation of two new baryonic strange beauty particles.

The LHCb collaboration submitted today a paper reporting the discovery of two new particles. The particles, known as the Ξ_b^'- and Ξ_b^*-, were predicted to exist by the quark model but had never been seen before.

Like the protons that the LHC accelerates, the new particles are baryons made from three quarks bound together by the strong force. The types of quarks are different, however: the new Ξ_b particles both contain one beauty (b), one strange (s), and one down (d) quark. Thanks to the heavyweight b quarks, they are more than six times as massive as the proton. But the particles are more than just the sum of their parts: their mass also depends on how they are configured. Each of the quarks has an attribute called "spin". In the Ξ_b^'- state, the spins of the two lighter quarks point in the opposite direction to the b quark, whereas in the Ξ_b^*- state they are aligned. This difference makes the Ξ_b^*- a little heavier.

The two new particles are observed through their decay into the ground state Ξ_b⁰ and a π^-. The image shows the distribution of δm, which is defined as the invariant mass of of the Ξ_b⁰π^- pair minus the sum of the π^- mass and the measured Ξ_b⁰ mass. This definition means that the lightest possible mass for the Ξ_b⁰π^- pair, known as the threshold, is at δm=0. The two peaks are clear observation of the Ξ_b^'- (left) and Ξ_b^*- (right) baryons above the hatched red histogram representing the expected background. Both particles have extremely short lifetimes and do not fly any measurable distance before they decay. But we can tell that the Ξ_b^*- is the more unstable of the two, since the peak is wider and the basic rules of quantum mechanics relate the average lifetime of a particle to the width of its mass peak (after taking into account experimental resolution). This is consistent with the pattern of masses: the Ξ_b^'- mass is just slightly heavier than the threshold, so it is allowed to decay into a Ξ_b⁰ and a π^- but only barely. The insert shows a zoom in the Ξ_b^'- δm region.

The masses, widths, production rates of these particles and more details on the analysis can be found in the LHCb publication. The result shows the extraordinary precision that LHCb is capable of: the mass difference between the Ξ_b^'- and the Ξ_b⁰ is measured with an uncertainty of 0.02 MeV/c², less than four one-millionths of the total Ξ_b⁰ mass. By observing these particles and measuring their properties with such accuracy, LHCb physicists make a stringent test of models of nonperturbative Quantum Chromodynamics (QCD). Theorists will be able to use these measurements as an anchor-point for future predictions.

Read more in the CERN press release in English and French, in the Research Highlights of Nature, in the Scientic American and in the Physics World news.

15 October 2014: φ_s: CP violation in the B_s system — looking for a chink in the armour of the Standard Model. Final run 1 result: φ_s = - 0.010 ± 0.039 rad

Today, at the workshop “Implications of LHCb measurements and future prospects” at CERN, LHCb physicists have presented their final run 1 results of their analysis of the phase φ_s (the B_s CP-violating phase, for experts), see 27 August 2012, 5 March 2012 and 3 March 2013 news for introduction. The measurement of φ_s is one of the most important goals of LHCb experiment.

The value of φ_s is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for B_s mesons, known to physicists as CP violation. The predicted value is small and therefore the effects of New Physics could change its value significantly.

In the CP violation that drives φ_s the role of B_s oscillations (3 March 2013 news) is very important. Here the Standard Model predicts very small effects, thereby allowing New Physics to manifest itself. This is to be contrasted with other manifestations of CP violation in the B_s system, unaffected by oscillations, where the Standard Model expects large signatures. Such a signature was already observed by LHCb, see 24 April 2013 news.

The 96 000 B⁰_s → J/ψ K⁺K^- decays collected during 2011 and 2012 data taking periods were used in the analysis. The left image shows the J/ψ K⁺K^- invariant mass spectrum. The very clean enhancement at the B⁰_s mass is clearly seen.

The right image shows that the K⁺K^- invariant mass spectrum is dominated by the presence of the φ meson resonance at a mass of 1020 MeV. The φ mesons, and similarly the J/ψ and ϒ mesons, are sometimes called heavy photons since they have the same quantum numbers as the photon. In their analysis LHCb physicists took into account different polarization states of the φ meson in analogy to different photon polarization states. For the first time, the phase of φ_s is measured independently for each polarization state of the K⁺K^- system. No significant difference is observed between the different polarisation states.

The result φ_s = - 0.058 ± 0.049 ± 0.006 rad is the most precise measurement of φ_s to date. Recently the LHCb Collaboration has reported the final run 1 measurement of the φ_s using B⁰_s → J/ψ π⁺π^- decays, φ^ππ_s = + 0.070 ± 0.068 ± 0.008 rad, consistent with the φ_s measurement reported above. The results from the two analyses have been combined giving the final value of φ_s = - 0.010 ± 0.039 rad. This is the most precise value of φ_s and is consistent with the Standard Model prediction of φ_s = - 0.0363 ± 0.0013 rad. The parameter region in which New Physics could still hide is now even smaller than before.

The LHCb today's result has been included in the HFAG world combination. The φ_s results of different experiments are shown in the image together with the ΔΓ_s variable (see 5 March 2012 news for introduction). The green ellipse in the center represents the LHCb result and the grey ellipse shows the world average. The result of the first φ_s measurement in the B_s → D_s⁺D_s^- decays φ_s = + 0.02 ± 0.17 ± 0.02 rad is also included in the LHCb combination shown in the image.

Read more in the LHCb workshop presentation and in the LHCb publication.

click the images for higher resolution

10 October 2014: Celebrating 200^th publication.

The LHCb Collaboration has celebrated this week its 200^th publication! The paper “Test of lepton universality using B⁺ → K⁺l⁺l^- decays“ appeared in Physical Review Letters on October 6^th. The American Physical Society, publisher of the Journal, has selected this article to be presented in “spotlighting exceptional research” news.

The results published today were already presented at the Large Hadron Collider Physics (LHCP) Conference in New York, see 3 June 2014 news and the Symmetry article.

7 October 2014: How bright is the LHC? LHCb has the most precise answer.

The LHCb Collaboration has just published the results of the luminosity calibration with a precision of 1.12%. This is the most precise luminosity measurement achieved so far at a bunched-beam hadron collider (see below).

At the beginning of operation of a new particle collider, physicists start with measurements of probabilities of different physical processes and compare results with theoretical predictions. Differences may indicate signs for new physics or needs for an improved understanding at higher energies of already known processes. The probabilities are measured as the so called “cross-sections”, σ, in units of area. The number of events N measured in an experiment depends on the value of cross-section σ (given by the nature of the physical interaction) and on a collider parameter luminosity L, N=σL . The luminosity L depends on the number of particles in each collider beam per unit of time and on the size of overlap of both beams at the collision point.

The formula N=σL indicates that the luminosity can be obtained from a measurement of a process for which the cross-section can be calculated with high precision. In this way a precision of few x 10^-4 was achieved at the e⁺ e^- collider LEP from the measurement of e⁺ e^- small angle scattering process whose rate was calculated precisely using QED theory. Cross-section predictions with similar or better precision are not available for proton-proton collisions. Therefore, purely experimental methods of luminosity measurements are used at the LHC. The number of particles in each colliding beam per unit of time (so called beam currents) are measured using LHC equipment. Two methods are used to measure the size of overlap of both beams at the collision point.

(1) The van der Meer scans (VDM) method, invented in 1968 by Simon Van der Meer, 1984 Nobel Prize winner, to measure luminosity in the Intersecting Storage Rings (ISR) at CERN, the world's first hadron collider. The idea was to measure the beams' overlap by scanning them across each other and monitoring the interaction rate. This method has been used by all four LHC experiments.

(2) The Beam-Gas Imaging (BGI) method was proposed by LHCb physicist Massimiliano Ferro-Luzzi in 2005. It takes advantage of the excellent precision of the LHCb Vertex Locator (VELO) placed around the proton-proton collision point. The BGI method is based on reconstructing vertices of interactions between beam particles and residual gas nuclei in the beam vacuum in order to measure the angles, positions and shapes of the individual beams without displacing them. This allows one to actually see the trace of the beams. To this date only the LHCb Collaboration is capable of using the BGI method.

The image shows the reconstructed positions of proton-beam-gas collision points. It is clearly seen that the proton beams are crossing at an angle. This is intentional since the protons in LHC are not continuously distributed around the ring but are concentrated in 1380 small regions called bunches (so the LHC is a bunched-beam collider). Colliding with an angle and not head-on assures that there are no unwanted collisions between bunches outside the region defined by the experiment. The luminosity was calibrated during special measurement periods and then relative changes were tracked by changes of counting rate of different sub-detectors. The vacuum pressure in the LHC was so low, that in order to increase the proton-beam-gas collision rate, LHCb physicists injected neon gas into the LHC vacuum tube during luminosity calibration periods.

Using the beam-gas data, LHCb physicists were able to reveal that a small fraction of the beam charge is spread outside the expected (i.e. "nominal") bunch locations. Since only collisions of protons located in the nominal bunches are included in physics measurements, it was very important to measure which fraction of the total beam current value measured with the LHC equipment participated in the collisions (i.e. contributed to the luminosity). Only LHCb could measure this fraction with sufficient precision and, therefore, the results of LHCb measurements of the charge fraction outside nominal bunch locations, called the "ghost" charge, were also used by the three other LHC experiments.

For proton-proton interactions at 8 TeV a relative precision of the luminosity calibration of 1.47% was obtained using van der Meer scans and 1.43% using beam-gas imaging, resulting in a combined precision of 1.12%. This represents the most precise luminosity measurement achieved so far at a bunched-beam hadron collider.

The LHCb BGI method was so successful that LHC engineers decided to use it also for the beam size measurements necessary for monitoring LHC operation. Dedicated equipment is now being built and will soon be installed in a specially modified region of the LHC ring. The equipment includes also a gas injection system.

The LHCb Collaboration is very well armed to measure precisely cross-sections of different processes at 13 TeV proton-proton collisions after the LHC restart in spring 2015.

Read more in the LHCb publication here and in the CERN Courier article.

29 September 2014: LHChamber Music.

Today, CERN, the European Organization for Nuclear Research, is blowing out 60 candles at an event attended by official delegations from 35 countries. Founded in 1954, CERN is today the largest particle physics laboratory in the world and a prime example of international collaboration, bringing together scientists of almost 100 nationalities.

To mark the occasion, music-minded physicists have transformed scientific data from the four underground detectors around CERN's Large Hadron Collider into a piece titled LHChamber Music composed by physicist-musician Domenico Vicinanza.

The video, presented during the official ceremony, can be viewed by clicking the arrow above.

The left image shows LHCb physicist Paula Collins, in front of the detector, playing music inspired by the data from the first observation of a heavy flavored spin-3 particle (15 July 2014 news).

The LHCb Collaboration has already presented another way of experimental data "sonification" in the 26 August 2013 news "Matter-antimatter quantum music".

11 September 2014: Combination of measurements of the CKM angle γ.

Today at the International Workshop on the CKM Unitarity Triangle, CKM2014, Vienna, Austria, the LHCb collaboration has presented a combination of measurements of the CKM angle γ (in tree decays, see below). For the first time a single experiment has achieved a precision of better than 10 degrees, which is better than the combination of the results of the B-factory experiments, BaBar and Belle. The parameters that describe the difference in behaviour between matter and antimatter, known as CP violation, are constrained in the so called CKM, or unitarity, triangle. The angles of this triangle are denoted α, β and γ, and of these it is γ that is the least precisely known. A detailed introduction to the CKM angle γ measurement can be found in the 5 October 2012 news and in the CERN Courier article. The measurement of the angle γ in different processes is one of the most important goals of LHCb experiment. The idea is to measure precisely the angle γ in processes in which a contribution from new physics is possible and in processes in which it is not. Comparison between the results of these two categories of measurement is therefore a powerful method to probe for the effects of new physics.

The value of the angle γ = (72.9^+9.2_-9.9)° presented today was obtained using B_(s)→D_(s)K^(*) decays in the analysis, in which B or B_s meson decays into D or D_s mesons were observed in the full 3 fb^-1 2011 and 2012 data set. The image shows a confidence level (CL) curve that indicates which values of γ describe best the LHCb data. The 68.3% horizontal line shows how the γ angle uncertainty was determined. Signs of new physics are not expected to show up in these decays (the so called tree-level measurements, for experts) and therefore they will set a base for comparison with the measurements where observation of new physics effects is possible.

The LHCb result will be improved still further before the start of new data taking period in spring 2015 using the already available data, since there are still important analyses to be completed. In addition LHCb has a large set of γ-sensitive observables in B→Dπ decays not discussed in this news. However their sensitivity to γ is suppressed compared to the B→DK-like decays.

Read more in the LHCb presentation in Vienna, in the LHCb conference note and in the CERN Courier article.

9 September 2014: Measurement of the semileptonic asymmetry, a^d_sl.

Today at the International Workshop on the CKM Unitarity Triangle, CKM2014, Vienna, Austria, the LHCb collaboration has presented the results of a measurement of the semileptonic asymmetry, a^d_sl, related to a difference between a probability of a beauty meson, B⁰, to oscillate into its antimatter partner, B⁰, and a probability of the reverse process (an introduction to beauty and charm oscillations can be found in the 7 November 2012 news item). Any difference in this probability would be a manifestation of what is called CP-violation. The label "d" indicates decays of B⁰ mesons composed of anti-beauty b and d quarks while "sl" (semileptonic) indicates that leptons, in this case muons, are present among decay products. When a B⁰ meson decays semileptonically, the charge of the lepton determines whether it was a decay of a matter B⁰ or anti-matter B⁰ meson. On the other hand a presence of a “wrong-sign” lepton in a decay of a B⁰ meson indicates that a transition to a B⁰ meson took place before decay. Therefore these "wrong-sign" B⁰ and B⁰ decays were used in the analysis. The measurement of the a^d_sl is very interesting since its value is predicted to be very small by the Standard Model and therefore any significant deviation from zero could indicate a (so called virtual) contribution of not yet discovered particles in B⁰ - B⁰ oscillations.

LHCb physicists presented today a new preliminary value of a^d_sl = (-0.02 ± 0.19 ± 0.30)% using the full 3 fb^-1 2011 and 2012 data sample. The LHCb Collaboration has published recently a corresponding value for the strange beauty mesons B⁰_s of a^s_sl = (-0.06 ± 0.50 ± 0.36)% using 1 fb^-1 of data taken in 2011. Both results indicate no CP-violation to be present within the sensitivity of the measurements and hence are consistent with the very small values predicted by the Standard Model.

The LHCb results are shown in the image together with the Standard Model prediction.

The yellow ellipse shows a result from a measurement of the D0 experiment at the Tevatron which is sensitive to both a^d_sl and a^s_sl. It lies a significant distance away from the prediction of the Standard Model and hence has excited interest as pointing to a break down in the theory, see 7 July 2012 news for introduction.

The LHCb results are consistent with the Standard Model but do not exclude the D0 result. LHCb will soon update the measurement of a^s_sl, which may clarify the situation.

Read more in the LHCb presentation in Vienna and soon in the forthcoming LHCb publication.

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15 July 2014: First observation of a heavy flavored spin-3 particle

Today at the 15^th International Conference on B-Physics at Frontier Machines at the University of Edinburgh, Beauty 2014, the LHCb collaboration has presented the results of a study of strange beauty meson B⁰_s decay into an anti-charm meson D⁰, a K^- meson and a π⁺ meson (B⁰_s → D⁰K^-π⁺). Previous results indicated the existence of a strange-charm D^*_sJ(2860)^- particle in the D⁰K^- invariant mass spectrum, and the study of the B⁰_s → D⁰K^-π⁺ decay allows one to study this structure and measure its properties. Today’s LHCb observation shows with 10σ significance that, in fact, this excess seen in the D⁰K^- mass spectrum is composed of two particles with different spins, spin-1 and spin-3. This is the first observation of a heavy flavored spin-3 particle, and the first time that any spin-3 particle has been seen to be produced in B decays.

The B⁰_s → D⁰K^-π⁺ decay is clearly identified as seen in the left image above. The right image above shows the enhancement at the 2.85 GeV/c² mass in the D⁰K^- invariant mass spectrum divided into two components as measured by LHCb physicists. The wider one corresponds to the spin-1 particle contribution and the narrow one represents the spin-3 contribution. The non-peaking distributions show contributions from other resonances that peak far from the 2.85 Gev/c² D⁰K^- mass region.

The left image shows to experts that the data D⁰K^- angular distribution (black points) is very well described by the presence of both spin-1 and spin-3 particles (solid blue curve). The models with only either a spin-1 (red curve) or a spin-3 (green one) particle are not supported by data.

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The D^*_sJ(2860)^- particles are composed of an anti-charm quark c and a strange quark s. The quark-anti-quark pair is bound by strong interactions and can form different quantum states with different values of spin and angular momentum in analogy to the different quantum states of ordinary atoms. The presence of the spin-3 contribution gives a clear signature that both particles are members of the so called 1D family having two units of angular momentum between the quark and the antiquark. This discovery demonstrates that the spectroscopy of the 1D families of heavy flavoured mesons can be studied experimentally. Further insights can be expected with similar analysis of B decays at LHCb and the LHCb upgrade.

Read more in the LHCb presentation, in the CERN Courier article and soon in the LHCb publication.

5 July 2014: First observation of Z boson production in proton-lead collisions

Today at the 37^th International Conference on High Energy Physics in Valencia, the LHCb collaboration has presented the first observation of Z boson production in proton-lead collisions at the LHC at a centre-of-mass energy per proton-nucleon pair of 5 TeV.

The main goal of lead-lead collision measurements at the LHC is to study the possible formation of a quark-gluon plasma state of matter in which quarks and gluons are freely moving instead of being bound inside protons and neutrons. According to the Standard Model of Cosmology this state of matter existed in the universe until 10^-5 seconds after the Big Bang. Although LHCb has not collected any data from lead-lead collisions, it has recorded data from proton-lead collisions. Studies performed on this data set make an important contribution to the interpretation of the possible signals of quark-gluon plasma production obtained in lead-lead collisions. LHCb has already shown interesting proton-lead results of this nature, as explained in the 10 May 2013 news.

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The first observation of Z production in proton-lead data presented today in Valencia confirms the importance of LHCb results in this field of research. The image shows the μ⁺μ^- invariant mass distribution. The Z boson peak is clearly visible and the background is negligible. The Z boson was discovered at CERN (together with the W⁺ and W^- bosons) in 1983 by the UA1 and UA2 collaborations and is the carrier of weak interactions.

The LHCb Z boson measurements are valuable for helping to determine the fraction of nucleon momentum carried by colliding partons (quarks and gluons) during the collisions of lead nuclei. The distribution of this fraction, usually termed "x", for partons in nucleons bound inside the atomic nucleus is different from the one for free nucleons. The Z bosons at LHCb are produced by the collisions of partons with small and large values of x allowing information to be learned about the behaviour of partons inside the nuclei both at very low and very large values of x. The capability to explore this kinematical range is an important and unique feature of LHCb experiment.

LHCb is planning to collect proton-lead data for a longer period during the new LHC run, which will allow more precise measurements of Z boson production, and other complementary studies, to be performed.

Read more in the LHCb conference presentation and in the LHCb publication.

3 July 2014: Unpredicted production mechanism of ϒ(3S) bottomium state.

Today at the 37^th International Conference on High Energy Physics in Valencia, the LHCb collaboration has presented important results on the production of beauty-anti-beauty quark pair bound states called Upsilon mesons, ϒ, in proton-proton collisions at the Large Hadron Collider LHC. The LHCb results include a precise measurement of the mass of one of these states, and more excitingly reveal the striking observation that about 50% of Υ(3S) mesons observed in LHC collisions are in fact not produced directly, but they originate from the radiative decay of χ_b(3P) mesons.

The beauty-anti-beauty bound state ϒ was discovered in 1977 at Fermilab near Chicago. Just like ordinary atoms bound by the electromagnetic force, the beauty and anti-beauty quarks, bound by the strong force, form different quantum states with different angular momenta and different spin orientations as discussed in the 6 September 2010 news. This atom-like system is called beauty quarkonium (or bottomium).

The different S quantum states (ϒ(1S), ϒ(2S), ϒ(3S)) have beauty quark and anti-quark spins aligned and no angular momentum L giving the total spin J=S=1. On the other hand the P states (χ_b(1P), χ_b(2P), χ_b(3P)) have angular momentum L=1. The χ_b(3P) state was recently observed by the Atlas and D0 collaborations. Today, the LHCb collaboration has presented the most precise measurement of its mass so far to be performed, and obtained the result 10511.3 ± 1.7 ± 2.4 MeV/c² (χ_b1(3P) mass for experts). The dotted red arrow labelled with "γ" indicates a possible decay path of the χ_b(3P): it may decay into a photon γ and an ϒ(3S) state. Similar transitions are possible to the ϒ(2S) and ϒ(1S) states.

The signal peaks coming from the S states decaying into muon pairs are shown in the left image below. The different states, ϒ(1S) , ϒ(2S) and , ϒ(3S) are very well separated thanks to the excellent resolution of the LHCb detector. LHCb physicists have then calculated the invariant mass of different ϒ states with different photons γ in order to find out if the ϒ(1S), ϒ(2S), ϒ(3S) states originate from the decay of different χ_b states, or not. This analysis showed that about 50% of the observed ϒ(1S) and ϒ(2S) states indeed come from the radiative decay of the χ_b particles, as many theorists had predicted. It was, however, generally assumed that the ϒ(3S) mesons were produced directly. Today's LHCb results show, however, that, in fact, also about 50% of the observed ϒ(3S) mesons are not produced directly but they originate from the χ_b(3P) radiative decay as shown by the presence of the χ_b(3P) peak in the right image below.

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Theoretical physicists will need to take into account the new LHCb results in their calculation of bottomium states production in proton-proton collisions at the LHC. The differences in the rates of production of different ϒ states in proton-proton and lead-lead collisions at the LHC have been interpreted as possible evidence for the formation of quark-gluon plasma in lead-lead collision. This interpretation will now have to be reassessed, taking account of the new LHCb results.

Read more in the LHCb conference presentation and in the LHCb publication.

1 July 2014: Guy Wilkinson and Monica Pepe Altarelli – new management for the LHCb Collaboration

Guy Wilkinson from the University of Oxford begins today his 3-year tenure as LHCb spokesperson. He replaces Pierluigi Campana from the Istituto Nazionale di Fisica Nucleare in Frascati. Monica Pepe Altarelli from CERN will play the role of deputy spokesperson during the same period by replacing Roger Forty and Burkhard Schmidt.

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Guy and Monica will face the huge challenges of run 2 and the following long technical shutdown during which LHCb will undergo a major upgrade. In the meantime, the discovery of new physics could be a dream within reach.

Pierluigi, Roger and Burkhard – thank you for your excellent work.

Guy and Monica – good luck.

Read more details at the LHCb Collaboration Web page and in the CERN Bulletin article in English and in French.

17 June 2014: No diploma without LHCb Vertex Locator

The baccalauréat, often known in France colloquially as le bac, is an academic qualification which French and international students take at the end of the lycée (High School) (secondary education). It is the main diploma required to pursue university studies.

Today French students had to pass the examination in physics and chemistry. In the first exercise the French students were calculating collisions at the LHC. Not only was the importance of Higgs boson discovery discussed but also the Beauty meson production inside the LHCb VErtex LOcator (VELO). The VELO detector is composed of two halves placed on both sides of proton beams around the proton-proton collision point. A VELO half is shown in the image.

If you understand French you can try to pass the French baccalauréat exam here.

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3 June 2014: An interesting result presented at the LHCP conference.

The LHCb Collaboration has just presented at the Large Hadron Collider Physics (LHCP) Conference in New York an interesting result: the ratio R_K of the probability that a B⁺ meson decays to a K⁺μ⁺μ^- or a K⁺e⁺e^- was measured to be different from one with a 2.6σ significance. The R_K is the ratio of two rare processes that occur twice in 10 million events (2x10^-7) representing b→s processes highly sensitive to the presence of virtual particles that only exist in extensions to the Standard Model.

In the Standard Model of particle physics this ratio is expected to be very close to one thanks to the so called lepton universality: leptons (like electrons e and muons μ here) behave in the same way (have the same couplings to the gauge bosons, for experts). The small differences in decay rates, tested so far with high accuracy, related mainly to the differences of their masses, are well understood. In the case of two decays discussed here the difference is expected to be at the per-mille level.

The ratio R_K is measured in function of the μ⁺μ^- and e⁺e^- invariant mass squared q². The LHCb result of R_K = 0.745^+0.090_-0.074 ±0.036 measured in the q² range between 1 and 6 GeV² is shown in the image as the black point with error bars together with the results from other experiments. It is interesting to note that the result of BaBar Collaboration at low q² shown by the red left point favors also a value below one. The blue point shows that the result of the R_K measurement by the Belle Collaboration is consistent with one in the whole q² range up to 22 GeV². The Belle result does not, however, contradict a possible q² dependence of R_K which may be indicated by the BaBar results. As seen in the image the LHCb measurement is the most precise measurement of R_K to date.

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Studies of R_K could detect the existence of additional Higgs bosons, new scalar or new pseudo-scalar particles that violate lepton universality, or “heavy Z”, Z’, which was suggested as a possible explanation of deviations observed in the P’₅ distribution in the analysis of B⁰→K^*0μ⁺μ^- decays, see 9 August 2013 news.

The pp collision data corresponding to 3 fb^-1 of integrated luminosity collected by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV were used to obtain this result. It will be interesting to follow up with future LHCb measurements of R_K at the 13 TeV pp collisions after the LHC restart planned for April 2015.

Read more in the LHCb presentation at the LHCP conference here, at the CERN public page here, in the Symmetry article, in the LHCb publication here, in the APS Physics Viewpoint and in the theorist's comment in CERN Courier.

24 April 2014: Studying properties of exotic particles.

The LHCb Collaboration has published today a paper which shows that the f₀(500) and the f₀(980) mesons, long thought to be four quark states (tetraquarks) made out of the light udud quarks (f₀(500)) or susu quarks (f₀(980)), are NOT consistent with being tetraquarks. The four quark states cannot be classified within the traditional quark model in which the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). They are therefore called exotic particles.

The Collaboration has studied the B⁰→J/ψπ⁺π⁻ decays. The left image below shows that the π⁺π⁻ invariant mass distribution is well understood as a contribution of different resonances and background distribution. The broad contribution of f₀(500) is clearly seen in the data. Note that the narrow K⁰_s peak structure is seen in the same mass region. There is no, however, evidence for the f₀(980) contribution. The f₀(980) production is much smaller than that predicted for tetraquarks and is ruled out at the 8σ level. The tetraquark model predicts a much smaller difference in the production rates of the two f₀ mesons. The right image below shows the corresponding π⁺π⁻ invariant mass distribution for the B⁰_s→J/ψπ⁺π⁻ decays. The contribution of f₀(980) is clearly visible. The absence of f₀(980) in B⁰ decays and its presence in B⁰_s decays as well as the presence of f₀(500) in B⁰ decays and its absence in B⁰_s decays is exactly what is expected if these states are normal qq states. Read more in the LHCb publication here.

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The precision of the LHCb detector and high production rate at the LHC allowed the LHCb Collaboration to made an important contribution to the understanding of exotic state candidates with heavy charm quarks inside, and to draw conclusions which of them are really exotic and which are not. Recently the Collaboration observed the Z(4430) particle and established its quantum numbers which make it the first confirmed unambiguous tetraquark (cucd), see 9 April 2014 news located just below at this page. This observation has implications to astrophysics and was followed up by an impressive media coverage.

The story of the X(3872) particle is more complicated. It was discovered about 10 years ago by the Belle Collaboration and since then was also studied by other experiments. The mass of 3872 MeV is located in a region where the pairs of charm (D) and anti-charm D mesons can be formed. In this mass region many charm quark-antiquark (cc) states are also present bound by a strong force in an atom-like system called charmonium. In addition the two different forms of exotic particles can be formed in this mass region: 1. About 10 fm large loosely bound DD* molecules in which D and D mesons are bound by a strong force in analogy to the deuterium nucleus composed of a proton and a neutron, and 2. About 1 fm compact four quark qqqq elementary particles, tetraqarks.

The LHCb has unambiguously determined the quantum numbers of X(3872) to be 1⁺⁺, see 26 February 2013 news . This are the quantum numbers of an as yet unobserved charmonium state called χ_c1(2³P₁). However, the charmonium spectrum is very well understood and the mass of the X(3872) makes this assignment very unlikely. The LHCb measurement of the ratio of branching fractions of X(3872) decay into ψ(2S)γ and J/ψγ does not support a pure DD* molecule interpretation of the X(3872), see 24 March 2014 news, and favours a hypothesis for an admixture of cc charmonium state and DD* molecule. However, the relativelly high production cross-section of the X(3872) at the LHC observed by the LHCb Collaboration, see 27 October 2010 news, favours even more an exotic tetraquark compact particle interpretation. More studies of X(3872) production and decay properties are needed to clarify its internal structure.

9 April 2014: Unambiguous observation of an exotic particle which cannot be classified within the traditional quark model.

The LHCb Collaboration has published yesterday results of precise measurements of properties of the Z(4430)^- particle which allow to determine unambiguously its exotic nature. In the traditional quark model, the strongly interacting particles (hadrons) are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particle physicists were searching since 50 years for the particles, called exotic hadrons, which could not be classified within this scheme. Many candidates have been proposed but up to now there has not been unambiguous proof of their existence.

The first evidence for the Z(4430) particle has been presented in 2008 by the Belle Collaboration as narrow peak in the ψ^’π^- mass distribution in the B → ψ^’Kπ^- decays. In the latest Belle publication the observation of the Z(4430) particle is confirmed with a significance of 5.2σ and a 3.4σ evidence is presented that the quantum numbers J^P = 1⁺ are favored over the other spin assignments. There are many so called charmonium cc neutral states in this mass region. The fact that the Z(4430) is a charged particle does not allow to classify it as a charmonium state making this particle an excellent exotic candidate. The BaBar collaboration could explain the Z(4430) enhancement in their data by a possible feature of experimental analysis (so called reflections, for experts), not contradicting in the same time the Belle evidence.

The LHCb Collaboration has reported yesterday an analysis of about 25 200 B⁰ → ψ^’Kπ^-, ψ^’ → μ⁺μ^- decays observed in 3 fb⁻¹ of pp-collision data collected at √s = 7 and 8 TeV. The LHCb data sample exceeds by an order of magnitude that of Belle and BaBar together. The significance of the Z(4430)^- signal is overwhelming, at least 13.9σ, confirming the existence of this state. The Z(4430)^- quantum numbers are determined to be J^P = 1⁺ by ruling out 0^-, 1^-, 2⁺ and 2^- assignments at more than 9.7σ, confirming the evidence from Belle. The LHCb analysis establishes the, so called, resonant nature of the observed structure in the data, and in this way proving unambiguously that the Z(4430) is really a particle.

The minimal quark content of the Z(4430) state is ccdu. It is therefore a four quark state or a two-quark plus two-antiquark state.

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The black points at the left image above show the ψ^’π^- invariant mass squared distribution of the data. The blue histogram shows the Z(4430) contribution. The right image shows the so called Argand diagram proving to the experts that the Z(4430) structure seen in the data (black points) represents really the resonant particle production and decay, since it follows approximately a circular path (red circle).

Read more in the LHCb publication here, in the CERN Quantum Diaries in English and French, in the CERN Courier article and also in the "Quarks bonding differently at LHCb" blog. See special video at CERN CDS or YouTube and the Z(4430) for saxophone quartet.

"A model-independent confirmation of the Z(4430)^- state" was submitted for publication in October 2015. A possibility of explaning the Z(4430) enhancement by the so called reflection of Kπ resonances, proposed by the BaBar collaboration, is excluded with a significance exceeding 8σ. Experts are invited to see the Fig. 9 and 12a and read the details in the paper.

Added in October 2016: Scientists will try to understand the “internal mechanism” of quark interactions inside tetraquarks. The two possibilities are illustrated in the figure. The color of the central part of each quark is related to the strong interaction color charge, while the external part shows its electric charge. The quarks could be tightly bound, or they could also be loosely bound in meson-meson molecule, in which color-neutral mesons feel a residual strong force similar to the one that binds nucleons together within nuclei. It is hoped that future measurements by LHCb will help to answer this question.

24 March 2014: New LHCb results at the Rencontres de Moriond.

The LHCb Collaboration has reported new important results at the Rencontres de Moriond EW Interactions and Unified Theories, March 15^th - 22^nd and QCD and High Energy Interaction, March 22^nd – 29^th.

The run 1 data taking period ended one year ago. The LHCb experiment collected 1fb^-1 of data from the pp collisions at 7 TeV in 2011 and additional 2fb^-1 at 8 TeV collisions in 2012. The results of analysis of beauty and charm particles decays were already presented at many conferences and reported in the news below. At this year Moriond conference more precise results of different analysis were presented using larger data sample and/or including other decay channels.

The decay of the beauty meson B into an excited K meson K^* and a μ⁺ and μ^- pair is considered as an important channel for new physics search, see 9 August 2013, 13 March 2012 and 22 July 2011 news for introduction. Different distributions and branching fractions have been studied for these B meson decays and compared with the Standard Model predictions. The differences in the results of measurements of neutral B meson decays into K^*0μ⁺μ^- and charged B⁺ meson decays into K^*+μ⁺μ^- is called an "isospin asymmetry". The Standard Model calculations predict this isospin asymmetry to be small what, in fact, was confirmed by the LHCb analysis of 2011 (1fb^-1) data. On the other hand, when the physicists made similar analysis by replacing the excited kaon K^* by its ground state K an evidence was obtained for a possible isospin asymmetry (see 25 May 2012 news). The analysis of full 3fb^-1 data sample presented at the Rencontres de Moriond gave results consistent with the small asymmetry predicted by the Standard Model in both (K^* and K) cases. However, even if the difference between results of measurements of neutral and charged B meson decays is small there is a tendency for differential branching fractions to have lower values than the theoretical predictions as seen in the images below.

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Exotic states, particles which are not composed of quark and anti-quark pairs (mesons) or three quarks (baryons), have been searched for since nearly 50 years. One of the most famous candidates for such an exotic state is called the X(3872). The LHCb Collaboration has unambiguously determined its quantum numbers J^PC to be 1⁺⁺ (see 26 February 2013 news for introduction). Possible exotic explanations of the X(3872) nature include a DD* molecules or multi quark anti-quark system such as a diquark-diantiquark tetraquark or charmonium-molecule mixture. Classical interpretation of X(3872) as a pure cc charmonium state is not excluded but this assignment is very unlikely (see again 26 February 2013 news).

The LHCb Collaboration has presented at the Rencontres de Moriond a result of measurement of ratio of branching fractions of X(3872) decay into ψ(2S)γ and J/ψγ, R_ψγ. This ratio is predicted to be different for different natures of X(3872) as seen in the image. The LHCb result 2.46 ± 0.64 ± 0.29 is more precise, but compatible with other experiments. It does not support a pure DD* molecule interpretation.

Read more in the LHCb presentations, LHCb papers and conference contribution here and in the CERN Courier article.

28 February 2014: First observation of photon polarisation in b→sγ transition.

The LHCb Collaboration has submitted today for publication a paper reporting the first observation of photon polarisation in b→sγ transition. The full 3 fb^-1 Run 1 data sample was used to obtain this result. The Collaboration has presented already the first evidence for the photon polarization in this process at the summer 2013 conferences using about 2/3 of the whole data sample, see the news of 19 July 2013 for an introduction.

Photon polarization is the quantum mechanical description of the classical polarized sinusoidal plane electromagnetic wave. Individual photon can have either right or left circular polarization or a superposition of both, read more here.

The beauty particles decay mainly into charm particles, less frequently into strange particles. About once in every 3000 decays into strange particles a photon is emitted. At the underlying quark level a beauty b quark turns into a strange quark s by emitting a photon γ. This famous b→sγ transition is considered as a very interesting process in which signs of new physics could show up. The first evidence for this process was obtained by the CLEO Collaboration in 1993 and since then it was intensively studied in many experiments. This decays occurs only rarely since it requires a quantum fluctuation where a pair of heavy particles (a top quark and a W boson) appear and then rapidly vanish. The interaction between these particles is such that the emitted photon is expected to be almost 100% (left-handed) polarized. However, since the “virtual” top and W particles are not seen in the detector, they could equally well be replaced by other even heavier particles that are predicted in various theories that go beyond the Standard Model. Such theories have been proposed to address important unresolved questions in particle physics, such as the origin of the imbalance between matter and antimatter seen in the Universe. These models generally predict different values for the photon polarisation, and therefore it is seen as one of the most important measurements that can be made with the latest generation of experiments.

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Researchers from the LHCb experiment have now succeeded to observe a non-vanishing value of the polarisation for the first time with a significance of 5.2σ. The analysis is based on nearly 14000 B⁺→K⁺π^- π⁺ γ decays, for which the distribution of the γ angle with respect to the normal to the plane defined by the kaon and two pion system is studied in four intervals of the K⁺π^- π⁺ mass which are shown in the left image. The two curves are fits to the data points, allowing photon polarisation (solid blue curve) or setting it to zero (dashed red curve). The right image shows a typical B⁺→K⁺π^- π⁺ γ event.

This investigation is conceptually similar to the historical Wu experiment that discovered parity violation by measuring the asymmetry of the direction of a particle emitted in a weak decay.

Read more in the LHCb publication here, in the CERN Courier article and in the CERN Bulletin article in English and French.

28 January 2014: How long can beauty and charm live together? [ τ( B_c⁺) = 509 ± 8 ± 12 fs ]

In the Standard Model of Particle Physics the strongly interacting particles (hadrons) are composed of three quarks (baryons) or quark-antiquark pairs (mesons). There are six types of quarks, three light: up (u), down (d) and strange (s) and three heavy: charm (c), beauty (b) and top (t). The top quark decays so fast that it cannot form bound particle states with other quarks. The heaviest quark which can form baryons or mesons with other quarks is the beauty quark, about five times heavier than proton. The particle physics theory predicts that the lifetime of particles made of the beauty quark and light quarks should be almost identical. The earlier experimental results were not supporting this prediction. Recently the LHCb Collaboration has shown, by measuring precisely the Λ_b lifetime, that indeed the theoretical predictions are correct, see 15 July 2013 news for details.

But what about the lifetime of particles composed of two heavy quarks? The LHCb Collaboration submitted today for publication a result of lifetime measurement of the B_c⁺ meson, formed of a b and a c quark. The lifetime of the B_c⁺ meson is measured using semileptonic decays having a J/ψ meson and a muon in the final state. The J/ψ mesons decay in turn into a pair of muons. The measured lifetime is 509 ± 8 ± 12 fs with an uncertainty less than half of that of the combination of results of previous measurements. This lifetime is much shorter than the lifetime of particles composed of a beauty quark and lighter quarks and closer to the lifetime of charmed particles formed by a charm quark and lighter quarks. This precise result is also very interesting for theorists studying decays of this kind of particles. The calculations are challenging since the effects of both strong and weak interactions need to be taken into account.

The image on the left shows the lifetime distribution of the B_c⁺ candidates, with the fitted components indicated. The analysis is performed on a data sample of pp collisions at a centre-of-mass energy of 8 TeV, collected during 2012 and corresponding to an integrated luminosity of 2 fb^-1. Further improvements are expected from the LHCb experiment using B_c⁺ → J/ψπ⁺ decays, where systematic uncertainties are expected to be largely uncorrelated with those affecting the present determination.

Read more in the LHCb publication and in the CERN Courier article.

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24 January 2014: “Underground” news.

The last run 1 collisions took place at LHCb on February 2013. The proton beams should traverse again LHCb in early 2015. The period in between is called Long Shutdown 1 (LS1). What is happening at LHCb, 100m underground, during this period?

”LHCb operated with great success throughout LHC run 1 and has not been subject to any major intervention since its assembly in 2008. The current long shutdown offers a first opportunity for prolonged access, and hence an extensive programme of consolidation and maintenance work has been scheduled. This programme involves all general and detector related services, equipment and safety systems” writes Rolf Lindner, the LHCb Technical Coordinator. Read more details here.

The left image shows the installation of the 30 tons shielding for the LHCb muon detector where 2100 blocks were piled up in a confined space. The right one shows the LHCb dipole consolidation.

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The LHCb underground cavern is also a very popular scientific “tourist” place. The total number of visitors in 2013 was 4524 for 392 visits guided by members of LHCb Collaboration. In addition about 2100 persons visited the LHCb detector and nearby LHC tunnel during CERN Open Days in September 2013. The CERN Visit Service showed the LHCb surface exhibition to 220 groups with 7935 visitors last year.

If you have not visited LHCb detector you can visit it “virtually” using the links from the LHCb public page (right column). The links include a Google Street View tour.

16 December 2013: LHCb VErtex LOcator VELO.

At LHCb the protons collide inside the VErtex LOcator detector (VELO). The VELO is composed of two halves, each consisting of 21 pairs of back-to-back silicon sensors, whose job is to precisely measure the position at which the charged particles pass (see left image below). You can watch the sophisticated construction process at the video here and can read more details about this detector here. Its sensitive detector elements are held out of harm's way while the beams are being injected and stabilized, but once safer, the silicon elements are moved mechanically in towards the beam to hunt for beauty and charm particles.

The LHCb VELO detector plays an essential role in locating precisely the pp collision point as well as the location of beauty and charm particle decays, see, for example, 12 March 2013 news. This detector is so important that the LHCb Collaboration decided to build a second identical Vertex Locator in order to replace the one located around beam should this ever be needed. Since the primary VELO detector is still working perfectly the replacement version is now displayed at the LHCb surface exhibition as seen in the right image below.

The LHCb Collaboration is working towards a major upgrade of the LHCb experiment for the restart of data-taking in 2019. Most of the subdetectors and electronics will be replaced so that the experiment can read out collision events at the full rate of 40 MHz. The upgrade will also allow LHCb to run at higher luminosity and eventually accumulate an order of magnitude more data than was foreseen with the current set-up.

The VELO performance will also be strongly improved. Pixel technology will be used to buid this third version of the detector. The new detector will contain 40 million pixels, each measuring 55 μm square. The pixels will form 26 planes arranged perpendicularly to the LHC beams over a length of 1 m (see image). The sensors will come so close to the interaction region that the LHC beams will have to thread their way through an aperture of only 3.5 mm radius.

Read more in CERN Courier article and in the LHCb VELO Upgrade Technical Design Report.

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29 October 2013: New charm results. [ x'² = (5.5 ± 4.9)x10^-5 ; y' = (4.8 ± 1.0)x10^-3 ]
[ A_Γ(KK) = (-0.35 ± 0.62 ± 0.12)x10^-3 A_Γ(ππ) = (0.33 ± 1.06 ± 0.14)x10^-3 ]

The LHCb Collaboration has reported recently new important results on charm physics.

(1) Ten months ago, the LHCb Collaboration presented the first observation of the D⁰-D⁰ oscillations in which the D⁰ matter mesons turn into their antimatter partners. Contrary to the B⁰-B⁰ and B⁰_s-B⁰_s oscillations in which the mesons turn into their antimatter partners many times during their lifetime, the D⁰-D⁰ oscillations are very slow, over one hundred times the average lifetime (see 7 November 2012 news for introduction). LHCb has now updated this result using the full 2011 and 2012 data set of 3 fb^-1. The new result is 2.5 times more precise. The values parameterizing the oscillations, the so-called mixing parameters y' and x'², are shown above.

By now, CP violation, differences in the behaviour of matter and antimatter, has been observed in all oscillating neutral-meson (K⁰, B⁰, B⁰_s) systems apart from the charm system. First evidence for charm CP violation (see 14 November 2011 news) has not been unambiguously confirmed to date (see 12 March 2013 news). The D⁰ mesons are the only mesons containing up-type quarks which undergo matter anti-matter oscillations (called also mixing) and therefore provide unique access to effects from physics beyond the Standard Model.

As part of the new analysis, LHCb has investigated whether there is a CP violating contribution to the oscillations, in contrast to the Standard Model expectation. This is done by investigating whether the oscillation parameters for mesons produced as D⁰ and D⁰ differ. Studying the D⁰ and D⁰ decays separately shows no evidence for CP violation and provides the most stringent bounds on the parameters (A_D and |q/p| for experts) describing this violation from a single experiment.

(2) LHCb physicists measured the asymmetry A_Γ of the inverse of effective lifetimes in decays of D⁰ and D⁰ mesons to the K^- K⁺ and π^-π⁺ final states. The measured values of the parameter A_Γ shown above represent the world’s best measurements of this quantity, and are the first searches for CP violation in charm oscillations with sensitivity better than 10^-3. They do not indicate CP violation, and show no difference in A_Γ between the two final states.

The results of other experiments combined by the Heavy Flavor Averaging Group indicated a hint for possible non-zero values of the CP violation parameters (|q/p| and φ for experts). Both LHCb results presented above do not support this indication as seen in the image. The size of the contour with the new LHCb results is about a factor of two smaller in each of |q/p| and φ. They provide very stringent limits on the underlying parameters, thus constraining the room for physics beyond the Standard Model.

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Read more in the LHCb presentations at the 6^th International Workshop on Charm Physics Manchester, England, and in the paper here and here and also in the CERN Courier article.

8 October: 2013 Nobel prize in Physics.

The Nobel Prize committee of the Royal Swedish Academy of Sciences has decided to award the Nobel Prize in Physics for 2013 to François Englert and Peter Higgs for "the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN's Large Hadron Collider".

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Congratulations to François Englert and Peter Higgs.

The image shows LHCb spokesperson Pierluigi Campana showing the LHCb detector to Peter Higgs on December 18th, 2012.

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28-29 September 2013: CERN Open Day at LHCb.

see CERN Open Day web page here for details.

2 September 2013: Excellent performance of LHCb Ring Imaging Cherenkov (RICH) detectors.

The physics results presented on this web page were obtained thanks to the excellent LHC collider performance, to the excellent LHCb data acquisition and analysis, and certainly also to the excellent quality of LHCb detector. As an example the exceptional precision of the particle identification achieved by one of the two RICH detectors is shown in the left image below. RICH detectors work by measuring the emission of Cherenkov radiation. This phenomenon occurs when a charged particle passes through a certain medium faster than light does. As it travels, a cone of light is emitted, which the RICH detectors reflect onto an array of sensors using mirrors. The shape of the cone of light depends on the particle’s velocity, enabling the detector to determine its speed. Scientists can then combine this information with a record of its trajectory (collected using the tracking system and a magnetic field) to calculate its mass, charge, and therefore its identity.

But why the speed of light is lower in the medium? - see an explanation here.

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The LHCb Collaboration has recently published a paper in the European Physics Journal C describing the performance of the LHCb RICH detectors. The Journal has honoured the excellent quality of LHCb detector by placing the RICH performance image on its cover page as seen in the right image above.

Read more about the LHCb detector at this web site here.

26 August 2013: Matter-antimatter quantum music.

A fascinating feature of quantum mechanics, in which the B⁰_s, B⁰ and D⁰ particles turn into their antimatter partners and back, has been discussed already few times at this page, see 3 March 2013 and 7 November 2012 news. This feature is called oscillations or mixing. The B⁰_s mesons oscillate with by far the highest frequency of about 3 million million times per second (3*10¹²), on average about 9 times during their lifetime. The B⁰ mesons oscillate about 37 times slower with a frequency of about 80 thousand million times per second (8*10¹⁰). A musical tone is defined by its frequency. LHCb physicists tried to hear this beautiful (involving beauty quarks) matter-antimatter quantum music.

The left video image above shows the last stage of the event filtering process. Two accumulations of events are clearly visible allowing to select the B⁰ and B⁰_s particles. As the blue box moves through the image we are able to hear the background noise, then the loud tone of B⁰-B⁰ oscillations, the background noise again and then the tone of the B⁰_s-B⁰_s oscillations. The higher frequency B⁰_s-B⁰_s oscillations are experimentally more difficult to observe and therefore their tone is weaker. The very high-pitched quantum oscillation frequencies were reduced by millions of times in order to fit into the range that can be heard by humans. Additional explanations can be found in the right hand side video above.

Read also the CERN public page news. Experts can read more about the LHCb analysis, event selection and oscillation parameter measurements in a recent publication here.

9 August 2013: LHCb results hint at new physics?

The LHCb Collaboration has just published the results of a new analysis of the B⁰→K^*0μ⁺μ^- decay, with K^*0→K⁺π^-. These results were presented three weeks ago at the European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, and triggered very interesting discussions. The analysis of the B⁰→K^*μμ decay is considered as a very promising channel to search for new physics effects, see the 14 June 2013 news for an introduction. A contribution from new physics particles could modify the angular distributions of the decay products. LHCb physicists have studied different variables related to these angular distributions as functions of the μ⁺μ^- invariant mass squared. In previously published results, no significant deviation from the Standard Model prediction has been found, see the 13 March 2012 news. In order to increase sensitivity to new physics effects LHCb physicists started to analyse additional observables (the so called P_i' observables) which are considered theoretically clean. This means that they are less sensitive than other observables to some theoretical parameters that are not precisely known (form-factors for experts). Four such observables, labelled P₄', P₅', P₆' and P₈', have been studied.

The image shows the distribution of the P₅' observable as a function of the μ⁺μ^- invariant mass squared q². The black data points are compared with the Standard Model prediction. A 3.7σ deviation of data above the prediction is observed for the third bin corresponding to q² between 4.3 and 8.68 GeV²/c⁴. Taking into account that this deviation is observed in one out of 24 bins investigated in this work (the so-called look-elsewhere effect), the significance of the deviation becomes 2.8σ.

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These new results are of great interest to theorists, who are combining results from several measurements to search for effects of physics beyond the Standard Model. According to Joaquim Matias from Universitat Autonoma de Barcelona and colleagues the deviation in P₅' and small discrepancies in the other angular observables for this decay, follow a pattern. In a recent paper the authors claim that a global analysis of the LHCb data, together with previous measurements, show a deviation of 4.5σ with respect to Standard Model expectations, which can be explained with the same mechanism (reduced Wilson coefficient C9 for experts). This demands further investigation, in particular to re-evaluate all the sources of theoretical uncertainty, and to understand the effects of correlations between the experimental measurements. A deep interplay between experimental and theoretical analyses will be essential to confirm or refute the pattern of new physics suggested by the B⁰→K^*μ⁺μ^- anomaly.

The results presented so far are based on 1fb^-1 of data recorded from pp collisions at 7 TeV in 2011. Particle physicists are impatiently waiting for result of analysis of additional 2fb^-1 of data taken at 8 TeV in 2012.

Read more in the LHCb presentation in Stockholm. in the LHCb paper and in the CERN Courier article.

24 July 2013: The B⁰_s→μμ decay is observed.

The CMS and the LHCb Collaborations have announced today at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, that the B⁰_s→μμ decay is observed. The LHCb Collaboration presented already at this conference the measurement of the B⁰_s→μμ branching fraction of (2.9^+1.1_-1.0)x10^-9 with a significance of 4.0σ (see the 19 July 2013 news). The CMS Collaboration presented the same day a similar result giving a branching fraction of (3.0^+1.0_-0.9)x10^-9 with a significance of 4.3σ (see CMS public page article). The results of both experiments are compatible and, therefore, a decision was taken to combine them.

The CMS and the LHCb Collaborations have obtained a combined preliminary value of the B⁰_s→μμ branching fraction of (2.9±0.7)x10^-9. Although a thorough evaluation of the combined significance has not been performed, it is clear that the B⁰_s→μμ decay is observed (with a significance above 5σ). The result is in agreement with the Standard Model prediction of (3.56±0.30)x10^-9. The image shows the CMS and LHCb results and their combination together with the results of the CDF Collaboration as well as the D0 and ATLAS Collaborations 95% CL limits which are not included in the combination.

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The search for the B⁰_s→μμ decay was considered as one of the most stringent tests of the Standard Model. Now it is found at a rate consistent to within 25% with that calculated within the Standard Model. This provides a fine-grained filter for the new physics models. All models of physics beyond the Standard Model will have to test their compatibility with this important result.

Read more in the plenary presentation in Stockholm, in the LHCb paper and also in the CERN Quantum Diaries in English and French.

19 July 2013: First evidence of photon polarisation in b→sγ transition.

The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, a first significant non-zero measurement of an observable proportional to the photon polarisation in b→sγ transition. The transition of a b-quark to an s-quark by emission of a photon (γ) is considered a very important process to investigate possible manifestation of new physics. This decay process is forbidden in the first approximation in the Standard Model (SM) of particle physics and moreover in the second-order processes that govern the process in the SM the emitted photon is expected to be strongly polarised. Therefore it is very sensitive to new physics effects arising from the exchange of new heavy particles in electroweak penguin diagrams (see 14 June 2013 news). Indeed, several models of new physics predict that the emitted photon should be less polarised than in the SM. Up to now different experiments have measured the decay rate of this process, ruling out significant deviations of the rate from the SM prediction and strongly reducing the allowed parameter space of new physics models. The photon polarisation was, however, never previously observed.

Free quarks are not observed in nature. Therefore physicists measure the b→sγ transition in decays of particles containing a b quark, like B mesons, into strange particles containing a s quark, like K mesons. The LHCb physicists have used the process B⁺→K_resγ where excited K meson states, K_res, decay in turn into three particles, K⁺, π^- and π⁺. The red distribution in the image shows the contribution of more than 8000 signal events reconstructed and selected in the 2012 data sample, corresponding to an integrated luminosity of about 2 fb^-1 collected in pp collisions at 8 TeV. The other distributions show different background contributions. An angular analysis of the B⁺ decay products has allowed to obtain first evidence, with 4.6σ significance, for photon polarization in the b→sγ transition with respect to the no-polarization scenario. Further theoretical analysis is, however, needed to obtain a numerical value for this polarization.

Read more in the LHCb presentation in Stockholm and in the LHCb conference note here.

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19 July 2013: Measurement of the B⁰_s→μμ branching fraction and search for B⁰→μμ decays at the LHCb experiment.

The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, improved measurements for the rare decays B⁰_s→μμ and B⁰→μμ. Last year at the Hadron Collider Particle Symposium in Kyoto, the Collaboration presented the first evidence, with 3.5σ significance, for the B⁰_s→μμ decay using the total 1.0 fb^-1 of data taken in 2011 and 1.1 fb^-1 of the data accumulated in 2012, see 12 November 2012 news. The full LHCb data sample of 3.0 fb^-1 was used to obtain today's result. The analysis strategy is very similar to that reported in November 2012 with an improved event selection algorithm (BDT). The significance of the result has been improved to 4.0σ making the evidence even stronger. A branching fraction of (2.9^+1.1_-1.0)x10^-9 is obtained. The result is in agreement with the Standard Model prediction of (3.56±0.29)x10^-9. It puts very strong constraints on the parameters of different models of new physics and squeezes even more than previous results the parameters of supersymmetric extensions of the Standard Model (SUSY) - see 30 March 2012 news for introduction.

The μ⁺μ^- invariant mass spectrum for the BDT selection algorithm bins with the smallest background contribution is shown in the left image. The solid blue line shows that the data distribution presented as black dots is well understood and can be separated into different components presented with the help of different colour lines. The dashed red narrow distribution shows the B⁰_s →μμ contribution around the B⁰_s mass of 5372 MeV/c². The green dashed line shows a possible B⁰ contribution. The B⁰ decay yield is not significant yet and an improved limit on the B⁰→μμ branching fraction of 7.4x10^-10 at 95% CL is obtained.

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A typical B⁰_s →μμ decay candidate event is shown below. The two muon tracks from B⁰_s decay are seen as a pair of purple tracks traversing the whole detector in the left image below. The right image shows the zoom around the proton-proton collision point, origin of many particle tracks. The two muon purple tracks originate from the B⁰_s decay point located 50 mm from the proton-proton collision.

Read more in the LHCb presentation in Stockholm and in the LHCb paper and in the CERN Press Release.

18 July 2013: Observation of unexpected resonant structure in B→Kμμ decays.

The LHCb Collaboration has just presented at the 2013 European Physical Society Conference on High Energy Physics, EPSHEP, Stockholm, Sweden, a first observation of unexpected resonant structure in B⁺→K⁺μ⁺μ^- decays. Precise study of this decay could uncover a possible contribution from new physics. This contribution could, however, hide behind the dominant B⁺→K⁺μ⁺μ^- decay modes which proceed through the decay of the B⁺ to a cc resonance (charmonium) and a K⁺ meson, followed by the decay of the resonance to a μ⁺μ^- pair. To probe for physics beyond the Standard Model it is necessary to remove regions of μ⁺μ^- mass dominated by the resonances. Up to now only the J/ψ and ψ(2S) resonances were taken into account because contributions from resonances with masses above 3900 MeV, where the kaon has a low recoil against the dimuon pair, were thought to be negligible.

The image shows the μ⁺μ^- mass distribution in the low recoil region. What was expected is a smoothly falling distribution, dominated by the non-resonant decay. However, two peaks are clearly visible, one at the low edge corresponding to the decay ψ(3770)→μ⁺μ^- and a wide peak at a higher mass. The mean and width of the wider peak are 4191⁺⁹_-8 MeV/c² and 65⁺²²_-16 MeV/c², which are compatible with the so-called ψ(4160) resonance (the name ψ(4160) is misleading, first measurements of the mass of this state gave values lower than it is now known to be). First observations of both the decay B⁺→ψ(4160)K⁺ and the subsequent decay ψ(4160)→μ⁺μ^- are reported with statistical significance exceeding six standard deviations.

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This observation is made possible due to quantum mechanical interference between the resonance and non-resonant signal. The resonance and the interference make up 20% of the yield in the low recoil region. This contribution is much larger than expected and in the future, with the large data sets available at LHCb, will need to be taken into acount when searching for new physics in rare decays such as B⁺→K⁺μ⁺μ^-.

Read more in the LHCb presentation in Stockholm and in the LHCb paper here.

15 July 2013: A mystery of the beauty baryon lifetime resolved. [ τ(Λ_b)/ τ(B⁰) = 0.976±0.012±0.006 ]
[ τ((Λ_b) = 1.482±0.018±0.012 ps ]

The LHCb Collaboration has just published an important precise measurement of the Λ_b beauty baryon lifetime. The Λ_b is a particle composed of the up (u), down (d) and beauty (b) quarks, so it can be understood as being like a neutron (composed of udd quarks) in which one of the d quarks has been replaced by the beauty (b) quark. Therefore the Λ_b baryon is about 6 times heavier than the neutron. The lifetime of the Λ_b baryon was first measured by experiments which took data at the electron-positron collider LEP in the 1990s. The results of the measurements were puzzling. It was a real nightmare for theoretical physicists. In fact, the calculations they were using, the Heavy Quark Expansion HQE, predicted that the Λ_b lifetime should be very similar to that of the B⁰ meson, but the LEP experiments found the Λ_b lifetime to be about 20% shorter than the B⁰ lifetime.

The LHCb Collaboration has recently discovered a new decay mode
Λ_b → J/ψpK^-, J/ψ→μμ. The image shows the signal yield of more than 15,000 Λ_b decays in 1.0 fb^-1 of LHCb data. This decay allows the precise measurement of the Λ_b decay point from the intersection of four charged tracks. The B⁰ decay point was also measured precisely using four charged tracks of the B⁰ decay into J/ψK^*0, K^*0→K⁺π^-. In this way the LHCb collaboration made the most precise measurement of the Λ_b to B⁰ lifetime ratio to be 0.976±0.012±0.006, close to 1 and in agreement with the original HQE prediction. The mystery of the Λ_b lifetime is now resolved. Using previous determinations of the B⁰ meson lifetime, the Λ_b lifetime was found to be 1.482±0.018±0.012 ps.

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Read more in the LHCb publication, in the CERN Courier article and in the CERN Quantum Diaries in English and French.

14 June 2013: LHCb: a place to find penguins.

How "penguins" can help LHCb to chase for new physics? - read explanations in the June issue of CERN Courier and then follow links to the LHCb physics results announced in the 13 March 2012 and 25 May 2012 news at this page.

The name of penguin appeared in particle physics in 1977 following a bet lost by CERN theoretical physicist John Ellis, see also explanations in the CERN Courier article. The original penguin diagram for b quark to s quark decay, where a gluon produces an ss pair, is shown in the image at the right hand side.

LHCb is investigating decays where a μ⁺μ^- pair is produced from a photon or from a Z boson as seen in the diagram on the left named as an "electroweak penguin"; the diagram on the right is a "box" diagram.

See also the hangout video,
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6 June 2013: LHCb PhD student wins the Swiss FameLab competition.

Donal Hill, LHCb PhD student, was nominated as the best Swiss FameLabber 2013 during the Swiss FameLab final in Zurich. Nine candidates selected in a previous heat in Geneva were evaluated by a judging panel and by the audience. Donal will represent Switzerland at the Cheltenham Science Festival in the UK in June 2013.

FameLab is an exciting competition for young researchers. It encourages scientists to inspire and excite public imagination with a vision of the 21st century of science.

Congratulations to Donal!

See Donal's presentation here, as well as the CERN bulletin article in English and French.

17 May 2013: LHCb physicist rewarded by the European Physical Society.

The High Energy Physics Division of the European Physical Society announced today the winners of its 2013 prizes, which will be awarded at the Europhysics Conference on High-Energy Physics (EPS-HEP 2013), Stockholm (Sweden) 18-24 July 2013. The 2013 Young Experimental Physicist Prize, for outstanding work by one or more young physicists in the field of Particle Physics and/or Particle Astrophysics, was awarded to Diego Martinez Santos “for his outstanding contributions to the trigger and commissioning of the LHCb experiment, and the analyses leading to first evidence for the rare decay B⁰_s →μμ”.

Congratulations to Diego!

The European Physical Society recognized in this way the quality and the excellence of the LHCb experiment, and of the whole collaboration who built and is operating it.

Read more on the analysis of B⁰_s →μμ data in the 12 November 2012 news.

10 May 2013: First and important results from the proton with lead ion collision 2013 run.

The LHCb Collaboration has just presented at the Workshop on proton-nucleus collisions at the LHC, Trento, Italy, the first results from the analysis of proton with lead ion collision run data taken in January-February 2013. Already these first results made an important contribution to the understanding of heavy ion collisions.

In the Standard Model of Cosmology quarks and gluons were freely moving in a state called a quark-gluon plasma until 10^-5 seconds after the Big Bang. As the Universe cooled, they became confined inside protons and neutrons. The theory of quark-gluon interactions, the strong force interaction theory, QCD, predicts that the state of quark-gluon plasma can also exists in high temperature matter created by high energy collisions between large atomic nuclei, called by physicists heavy ion collisions. But how to prove that the quark-gluon plasma is really formed? A reduced rate of J/ψ particle production in heavy ion collisions was considered as a "smoking gun" argument in favour of quark-gluon plasma formation by physicists analysing results of measurements performed in the CERN Super Proton Synchrotron (SPS) after 1986 and more recently in the Brookhaven RHIC collider. Profound analysis has shown, however, that reality is more complicated. In some models, for example, the J/ψ particle could also be regenerated in nuclear matter, partons (quark, gluons) could be saturated and/or lose energy, etc. in normal (so called cold) nuclear matter.

Data recording collisions of protons with lead ions were collected in the LHC experiments in January-February this year. In such collisions, formation of a quark-gluon plasma is not expected, and therefore measurements based on these data allow the study of interactions in cold nuclear matter. The analysis of J/ψ production was of particular interest.

The LHCb results are shown in the image at the left hand side. A reduced value of the nuclear attenuation factor R_pA, the ratio of the J/ψ production in the proton with lead ion (pA) collisions to that in proton-proton collisions as a function of the rapidity y is clearly seen. The rapidity variable is related to the J/ψ production angle with respect to the incoming proton direction. The experimentally measured points (triangles with error bars) in the image show that the largest suppression is in the forward direction.

The colored distributions show theoretical predictions of R_pA calculated by François Arleo from LAPTH, Annecy and Stéphane Peigné from Subatech, Nantes, taking into account the J/ψ energy loss (E. loss) in cold nuclear matter with and without the parton saturation effects. The LHCb results are in agreement with these predictions.

Two sets of data were taken: pA and Ap, where in the second case the direction of the proton and lead ion beams were reversed. This allowed the LHCb detector, recording the particles only on one side of the interaction point, to make measurements in both forward and backward directions with respect to the proton beam (positive and negative rapidity).

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Read more in the LHCb presentation in Trento and in the LHCb conference note.

24 April 2013: First observation of CP violation in the decays of B⁰_s mesons. [ A_CP(B⁰_s→K^-π⁺) = +0.27 ± 0.04 ± 0.01 ]
[ A_CP(B⁰→K⁺π^-) = -0.080 ± 0.007 ± 0.003 ]

The LHCb Collaboration has just submitted for publication a paper which sets an important milestone in the history of particle physics. A difference between properties of matter and antimatter, named CP violation by particle physicists, was discovered in 1964 in the decays of neutral K mesons and was rewarded with the 1980 Nobel Prize in Physics for James Cronin and Val Fitch. M. Kobayashi and T. Maskawa proposed in 1973 a mechanism which could incorporate CP violation within the Standard Model with not less than 6 quarks. In 2001, CP violation was observed in the decay of so-called Beauty Particles, the B⁰ mesons composed an anti-quark b and a quark d. The Standard model mechanism of CP violation was confirmed and therefore Kobayashi and Maskawa were rewarded with the 2008 Nobel Prize in Physics. In March 2012 the LHCb Collaboration reported an observation of CP violation in charged B^± meson decays into DK^±. Today, the LHCb Collaboration has announced an observation of CP violation in the decays of Strange Beauty particles, the B⁰_s mesons composed of a beauty antiquark b bound with a strange quark s.

The B⁰ and B⁰_s meson decays into K and π mesons were studied. The decay into a negative K meson red track and a positive π green track is show in the event displays above.

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The four plots on the left hand side above show the Kπ invariant mass distribution divided into different components as shown by the legend in the top-right figure. The different charge combination of K and π indicates if the decaying B⁰ or B⁰_s particle is a matter or an antimatter particle. The two upper plots show that the decay rates of B⁰ mesons are different, as was already well established by previous measurements. The zoom in the lower two plots shows that the difference is also visible around the B⁰_s meson mass, as indicated by the two green Gaussian distributions. Mathematically this difference is described by the asymmetry A_CP(B⁰_s→K^-π⁺) = +0.27 ± 0.04 ± 0.01, which differs from zero with significance exceeding five Gaussian standard deviations σ. Therefore this result represents the first observation of CP violation in the decays of B⁰_s mesons. The corresponding asymmetry for B⁰ meson decays presented in the two upper images, A_CP(B⁰→K⁺π^-) = -0.080 ± 0.007 ± 0.003, is the most precise measurement of this quantity to date.

The full 1.0 fb^-1 data sample collected in 2011 was used to obtain these results, the precision will be further improved using the total dataset available which has more than tripled thanks to the excellent 2012 data taking period.

More details can be found in the LHCb paper here. Read also the CERN Press Release in English and French, the CERN Bulletin article in English and French as well as CERN Courier article.

20 March 2013: LHCb 100th publication.

The LHCb Collaboration has submitted its 100th publication! It is signed by 621 authors from 63 different universities and laboratories from 17 countries. The paper "Search for direct CP violation in D⁰→h^-h⁺ modes using semileptonic B decays" has been presented at the Rencontres de Moriond QCD, La Thuile, Italy, and is described in the 12 March 2013 news as the second independent analysis. You can celebrate this important moment in the life of our Collaboration with the help of a poster available in the LHCb secretariat or printing it directly from a file. The poster shows an event, described in the 12 March 2013 news, and used in the analysis.

The LHCb papers have made very important contributions to particle physics as described in other items on this page. However, even more important contributions are expected in the near future. In fact, most results presented in LHCb papers to date used the full 1.0 fb^-1 data sample collected in 2011. The total dataset available for future analysis has more than tripled thanks to the excellent 2012 data taking period. The only published paper using the 2012 data is the analysis presented in the "First evidence for the B⁰_s →μμ decay" paper, see 12 November 2012 news, in which already half of 2012 data sample was used.

The highlights of recent LHCb results showing the presentations at different conferences, conference contributions and papers can be found here.

12 March 2013: Improved search for CP violation in charm decays. [ ΔA_CP = (−0.34 ± 0.15 ± 0.10 )%, pion tagged ]
[ ΔA_CP = (+0.49 ± 0.30 ± 0.14)%, muon tagged ]

The LHCb Collaboration presented today at the Rencontres de Moriond QCD, La Thuile, Italy, results of an improved search for the difference between properties of matter and antimatter, CP violation, in charm decays, see 14 November 2011 news for introduction. The difference (Δ) of CP asymmetry (A_CP) between the decay rates of D (matter) and D (antimatter) mesons into K⁺K^– pairs and into π⁺π^- pairs was measured. The results presented today profited from three improvements to the previous analysis: the full 1.0 fb^-1 data sample collected in 2011 was used instead of 0.6 fb^-1, the analysis technique was improved and also in addition another independent method was used to select matter D and antimatter D particle decays.

In the Standard Model CP violation was expected to be very small in the charm sector, whereas new physics effects could generate enhancements. Therefore the 14 November 2011 announcement by the LHCb Collaboration of 3.5σ evidence of CP violation in charm sector, ΔA_CP = (-0.82 ± 0.21 ± 0.11)%, triggered intensive theoretical activity with conclusions that some special Standard Model effects could generate CP violation effects even as big as about 1%. This interesting LHCb result was later confirmed by the CDF and Belle collaborations. The new improved LHCb result presented today, ΔA_CP = (−0.34 ± 0.15 ± 0.10 )%, is more precise thanks to the larger data sample and several improvements resulting in better background suppression by a factor of 2.5. The central value is, however, closer to zero than in the previous measurement, which it supersedes.

In the measurement presented above the D (matter) and D (antimatter) mesons were selected using the D^* meson decays, D^*+(-) → π^+(-)D(D), which means that the presence of π⁺ in the decay identified matter D meson production while π^- accompanied antimatter D production. LHCb physicists presented today also results of a second independent analysis in which the D and D mesons were selected using so called semileptonic beauty B decays, for example B^+(-) → μ^+(-)νD(D). In the second analysis, the positive charge of μ⁺ identified the D meson, while the negative one, μ^-, the D production. The image at the left hand side shows a selected event. A zoom around the pp interaction point shows a B⁺ meson decay point located at the distance of 17 mm from the pp collision point and the D meson decay place still 9 mm further away. The second analysis also measures a value that is consistent with zero: ΔA_CP = (+0.49 ± 0.30 ± 0.14)%. A combination of the two LHCb results gives ΔA_CP = (-0.15 ± 0.16)%.

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The image at the left hand side shows a comparison of different measurements of ΔA_CP. The previous LHCb result is shown as the shaded grey point. A naive world average is shown as the yellow band. As shown in the image, the two new LHCb results are consistent with each other and with other results at the 2σ level, but do not confirm the previous evidence of CP violation in the charm sector.

Theoretical work has shown that several well-motivated models could induce large CP violation effects in the charm sector. These new results constrain the parameter space of such models. Further update of this and related measurements will be needed to discover if – and at what level – nature distinguishes between charm and anticharm.

More details can be found in the LHCb presentation in La Thuile, in the LHCb paper and conference contribution. Read also the CERN Courier article. See update with the full Run 1 data sample in the 2014 paper and in the 2016 seminar, in the 2016 paper and CERN Courier article.

3 March 2013: Precise search for new physics. [ Δm_s = 17.768 ± 0.023 ± 0.006 ps^-1 ]
[ φ_s = 0.01 ± 0.07 ± 0.01 rad ]
ΔΓ_s = 0.106 ± 0.011 ± 0.007 ps^-1 ]

The LHCb Collaboration presented today at the Rencontres de Moriond EW, La Thuile, Italy, three important results of their more and more precise search for new physics. The 1 fb^-1 data sample collected in 2011 was used to obtain these results.

(1) A fascinating feature of quantum mechanics, in which the B⁰_s, B⁰ and D⁰ particles turn into their antimatter partners, has been discussed already at this page, see 15 March 2011 and 7 November 2012 news. This feature is called oscillations (mixing). The B⁰_s mesons oscillate with by far the highest frequency of about 3 million million times per second (3*10¹²), on average about 9 times during their lifetime.

The B⁰_s meson decays into D^-_sπ⁺ were used in this analysis with D^-_s decays into five different channels. The image at the left hand side illustrates the B⁰_s-B⁰_s oscillations in a spectacular way, showing how the matter turns into antimatter and back over many oscillation periods. The frequency of oscillation is defined by the Δm_s parameter. The value of this parameter as measured by the LHCb collaboration is shown above.

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(2 - presented also at the Rencontres de Physique de la Vallée d'Aoste) Knowledge of the value of the parameter φ_s is very important for physicists, since it set the scale of the difference between properties of matter and antimatter, called CP violation by experts, in the B⁰_s sector. The B⁰_s decays into a J/ψφ and a J/ψππ were already studied, see 5 March 2012 and 27 August 2011 news for introduction. LHCb physicists have improved and finalized these measurements. One important improvement is in "flavour tagging", which determines whether the initial state was produced as a B⁰_s or B⁰_s meson. Better tagging gives better sensitivity in the final result. The values of the φ_s parameter together with the difference between the width of a heavy and light mass B meson system (see 5 March 2012 news) are shown above. The left image below shows the small allowed region for these two parameters.

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(3) Finally, LHCb physicists have opened a door for important future measurements by presenting the results of a first study of the time-dependent CP-violating asymmetry in hadronic B⁰_s meson decays into an φφ pair. Both φ mesons decayed in turn into a K⁺K^- pair. The invariant mass spectrum for the four kaons is presented in the right image above showing a clear accumulation of B⁰_s → φφ decays at the B⁰_s mass. Using about 880 events the CP-violating phase φ_s was measured to be in the interval of [-2.46,-0.76] rad at 68% confidence level.

The results presented in this news represent the most precise measurements to date. They are in agreement with the Standard Model prediction. The parameter region in which the signs of new physics can still hide is significantly reduced. More details can be found in the LHCb presentations in La Thuile and in the LHCb papers here. Read also the CERN Courier article.

26 February 2013: X(3872) looks more and more exotic. [ X(3872) J^PC = 1⁺⁺ ]

The LHCb Collaboration presented today at the Rencontres de Physique de la Vallée d'Aoste, La Thuile, Italy, an important result which makes the exotic nature of the X(3872) particle very probable.

In the quark model of particle physics proposed in 1964 by Murray Gell-Mann and George Zweig mesons, like the π (pion), are formed from quark and anti-quark pairs and baryons (like the proton) from three quarks. This model is very successful. Particles which cannot be described in this model, known as exotic states, have been searched for since nearly 50 years ago. Their existence has not yet been firmly established (except of a special case of pionium). One of the most famous candidates for such an exotic state is called the X(3872). It was discovered in B⁺ meson decay into an X(3872) and a K⁺ meson by the BELLE collaboration almost 10 years ago. Its existence was confirmed later by the CDF, D0 and BaBar experiments. LHCb has previously reported studies of the X(3872) in the data sample taken in 2010, see 27 October 2010 news. Particles are classified according to their quantum numbers J^PC. An analysis by the CDF experiment has limited the possible values of X(3872) J^PC to either 1⁺⁺ or 2^-+.

LHCb physicists have observed the X(3872) particle in the decay of a B⁺ meson into an X(3872) and a K⁺ meson. The X(3872) was observed in the invariant mass of a J/ψ particle and a π⁺π^- pair, while the J/ψ was identified from its decay into μ⁺μ^- pair. The image at the left hand side shows the difference between the invariant mass of the π⁺π^-J/ψ combination and the J/ψ showing clearly the X(3872) and ψ(2S) enhancements over the smooth background distribution (the ψ(2S) particle decays also into a π⁺, π^-, and J/ψ).

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LHCb physicists made a sophisticated analysis of the whole B⁺ decay chain in 5 dimensions and unambiguously determined the quantum numbers of X(3872) to be 1⁺⁺. The other previously allowed assignment of 2^-+ was rejected with statistical significance over 8σ. About 300 signal events were selected among about 60 million million (6*10¹³) pp collisions seen by the LHCb detector at LHC during the 2011 data taking period. Details of analysis can be found on the LHCb staff page.

The exotic nature of the X(3872) would be unambiguously determined if its quantum numbers could not be described by the quark-antiquark combination. However, this is not the case. In fact, the mass of 3872 MeV is located in a region in which many charm quark-antiquark states are present bound by a strong force in an atom-like system called charmonium. The X(3872) has the quantum numbers of an as yet unobserved charmonium state called χ_c1(2³P₁). However, the charmonium spectrum is very well understood and the mass of the X(3872) makes this assignment very unlikely. Possible exotic explanations of the X(3872) nature include a DD* or multi quark anti-quark system such as a diquark-diantiquark tetraquark or charmonium-molecule mixture.

More details can be found in the LHCb presentation in La Thuile and in the LHCb paper here. Read also the CERN Courier article and the CERN Bulletin article in English and French.

14 February 2013: End of 2013 data taking period.

Today at 7:25 LHCb physicists have observed the last collision at the LHC. After a very succesfull period of proton with lead ion collisions the last few days of data taking were reserved for proton-proton collisions at 2.76 TeV. The two year shut-down period, called LS1, will start two days later in order to set-up LHC for doubling the proton-proton collision energy to 13 TeV at March 2015. LHCb Collaboration congratulates and thanks LHC team for excellent performance.

Read the CERN Bulletin article in English and French.

20 January 2013: Start of 2013 data taking period.

Today at 15:11 LHCb physicists have observed the first 2013 collisions of protons with lead ions in the LHCb detector. The 2013 data taking period has started. The proton - lead ion collisions were alredy observed by the LHCb on 13 September 2012 during the short test run.

15 November 2012: LHCb thanks LHC.

LHC collider has delivered at 9:50 today 2 fb^-1 of luminosity to LHCb this year corresponding to about 100 million million of proton-proton collisions visible at LHCb. This was possible thanks to an exceptional performance of the machine and an impressive commitment from everybody involved in the operation of LHC in this very long running year. The image shows how the LHCb Collaboration thanks the LHC operation team on the LHC status screens which are visible in the control rooms all over CERN and on the Web. The collider is currently running with very high efficiency. This can be seen in the image where very long proton-proton collisions periods are interleaved by short set-up intervals.

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12 November 2012: First evidence for the B⁰_s →μμ decay. [ Branching ratio B⁰_s →μμ = (3.2^+1.5_-1.2)x10^-9 ]

The LHCb Collaboration has presented today at the Hadron Collider Particle Symposium in Kyoto the result of the branching ratio measurement of the B⁰_s meson decay into μ⁺μ^- pair to be (3.2^+1.5_-1.2)x10^-9. Both experimental and theoretical physicists were impatiently waiting for this result, an important milestone of the LHCb program. The significance of the measurement is 3.5σ and therefore is classified as the first evidence for the B⁰_s →μμ decay. The result is in agreement with the Standard Model prediction of (3.54±0.30)x10^-9 (for experts: this number takes into account a correction to the value (3.23±0.27)x10^-9 due to the finite width difference of the B⁰_s system). LHCb physicists had previously presented 15 March this year the lowest published limit of 4.5x10^-9 for this decay, which allowed to squeeze strongly the parameters of supersymmetric extensions of the Standard Model (SUSY) - see 30 March 2012 news. The measurement presented today squeezes the parameter space even more.

The total 1.0 fb^-1 of data taken in 2011 and 1.1 fb^-1 of the data accumulated this year were used to obtain this result. A special event selection (BDT for experts) was used to classify data into bins with different ratios of B⁰_s →μμ decays and background contributions. The μ⁺μ^- invariant mass spectrum for the bins with the smallest background contribution is shown in the left image. The solid blue line shows that the data distribution presented as black dots is well understood and can be separated into different components presented with the help of different colour lines. The dashed red narrow distribution shows the B⁰_s →μμ contribution around the B⁰_s mass of 5366 MeV/c².

The green dashed distribution shows a possible contribution from the B⁰ →μμ contribution around the B⁰ mass of 5280 MeV/c². Within the Standard Model the branching ratio for this decay is expected to be about 30 times smaller than that for the B⁰_s decay. A small excess of data over the background and Standard Model rate is observed, but is consistent with the Standard Model expectation. LHCb physicists have set a limit of 9.4x10^-10 for this branching ratio.

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A typical B⁰_s →μμ decay candidate event is shown above. The two muon tracks from B⁰_s decay are seen as a pair of purple tracks traversing the whole detector in the left image above. The right image shows the zoom around the proton-proton collision point, origin of many particle tracks. The two muon purple tracks originate from the B⁰_s decay point located 14 mm from the proton-proton collision.

The precision of these results will be improved using an additional 1 fb^-1 of data or more that will be available by the end of this year thanks to the strong and continuous support from the LHC operations team for the LHCb physics program.

More details can be found in the LHCb presentation in Kyoto and in the LHCb paper here and also in the CERN seminar. Read also the CERN Bulletin article in English and French, the CERN Courier article, the CERN Quantum Diaries blog in English and French and in the Scientific American blog.

7 November 2012: Oscillating charm and beauty. [ Δm_d = 0.5156 ± 0.0051 ± 0.0033 ps^-1 ]
[ x'² = (-0.9 ± 1.3)x10^-4 ; y' = (7.2 ± 2.4)x10^-3 ]

A fascinating feature of quantum mechanics has been reported 15 March 2011 on this page. The strange beauty particle (matter) B⁰_s composed of a beauty antiquark (b) bound with a strange quark s turns into its antimatter partner composed of a b quark and an s antiquark (s) with a frequency of about 3 million million times per second (3*10¹²). This feature is called "oscillations" or "mixing". The LHCb Collaboration has just published the first observation of similar oscillations of charm mesons D⁰ composed of a charm quark and an anti-up quark (D⁰-D⁰ oscillations) and the most precise measurement of a parameter defining the frequency of the oscillations of beauty mesons B⁰ composed of a beauty antiquark (b) bound with a d quark (B⁰-B⁰ oscillations).

The B⁰ decays into D⁺π^- and J/ψK^*0 were used to study B⁰-B⁰ oscillations. The images below show the asymmetry which is proportional to the difference between the number of events in which the matter (or antimatter) B⁰ particle decayed with the same flavour identity with which it was produced, and the number of events in which it did not, as a function of its lifetime. The B⁰-B⁰ oscillations are clearly visible. LHCb physicists have parametrized them with a value Δm_d = 0.5156 ± 0.0051 ± 0.0033 ps^-1 corresponding to the frequency of about 80 thousand million times per second (8*10¹⁰), about 37 times slower than B⁰_s-B⁰_s oscillations. The B⁰-B⁰ oscillations have been previously measured at LEP, Tevatron and B factories. The LHCb result is currently the most precise measurement of the Δm_d parameter.

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D⁰-D⁰ oscillations are very slow, over one hundred times the average lifetime of a D⁰ meson, so the full oscillation period cannot be observed. Instead, it is necessary to look for small changes in the flavour mixture (matter or antimatter) of the D⁰ mesons as a function of the time at which they decay. One of the best channels to search for this mixing is the D⁰ decay to the Kπ final state. The initial matter-antimatter identity can be identified by the charge of the accompanying pion in the decay D^*+→D⁰π⁺ or D^*-→D⁰π^-. The mixing effect (oscillation) appears as a decay-time dependence of the ratio R between the number of reconstructed “wrong-sign” (WS; D⁰→K⁺π^-) and “right-sign” (RS; D⁰→K^-π⁺) processes. In the absence of mixing, R is predicted to be constant as a function of the D⁰ decay time t, while, in case of mixing, it is predicted to be an approximately parabolic function of t. The left image below shows the WS over RS ratio R, as a function of decay time, from a total of 36 thousand WS and 8.4 million RS decays selected from the 1.0 fb^-1 of data recorded in 2011. The horizontal dashed line shows the no-mixing hypothesis, the solid line is the best fit to data when mixing is allowed. The clear time dependence observed excludes the no-mixing hypothesis by 9.1 σ.

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Comparison of values parameterizing the D⁰-D⁰ oscillations, y' and x'², obtained by different experiments, are presented in right image above. The LHCb results exclude the no-mixing hypothesis by more than 5 σ for the first time and therefore can be classified as the first observation of this effect.

Since the Standard Model predictions for the mixing parameters have large uncertainties, the next step will be to focus on cleaner observables to search for possible new physics contributions. In particular, LHCb is now well placed to investigate whether there is a CP violating contribution to the oscillations, in contrast to the Standard Model expectation. This will be achieved by studying charm mixing in this and other decay channels and exploiting the large increase in available statistics following the successful 2012 LHC run.

Read more in the LHCb publications: D⁰-D⁰ and B⁰-B⁰ oscillations, in the CERN Courier article and in the USLHC Quantum Diaries blog.

5 October 2012: Measurement of the γ angle. [ γ = (71.1^+16.6_-15.7)° only B^±→DK decays ]
[ γ = 85.1° uncertainty regions [61.8,67.8]° and [77.9,92.4]°, B^±→DK and B^±→Dπ decays ]

LHCb is an experiment set up to explore what happened after the Big Bang that allowed matter to survive and build the Universe we inhabit today. Therefore LHCb physicists are measuring differences between properties of matter and antimatter, called CP violation by experts. CP violation was discovered experimentally in K meson decays in 1964. M. Kobayashi and T. Maskawa proposed in 1973 a mechanism which could incorporate CP violation within the Standard Model with not less than 6 quarks; they were awarded the Nobel prize of Physics in 2008 for this idea. The size of this violation is set by the parameter η, which is shown as the y axis in the figures below. Constraints on η and the related parameter ρ (the x-axis) are measured in various ways in different experiments as shown in the compilation made by the CKMfitter group in the left image below. The constraints show that in fact the values of ρ and η within the small colored region in the center of the images are compatible with the experimental results and confirm in this way the Kobayashi and Maskawa Standard Model mechanism of CP violation. However, since this mechanism does not explain the large quantity of matter observed in the Universe physicists are searching for other sources of CP violation outside the Standard Model.

An interesting possibility is to measure precisely the angle γ of the triangle shown in the right image below in processes in which the new physics contribution is possible and in processes in which it is not. Differences between measurements in these two cases would be a sign of new physics. The measurement of the angle γ in different processes is one of most important goals of the LHCb experiment. The value of this angle is known up to now with precision of only about 10 or 12°, as seen in the right image below, using the combination of results from other experiments.

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LHCb physicists have just presented at the 7th International Workshop on the CKM Unitarity Triangle, Cincinnati, Ohio, USA, the measurement of the angle γ in processes where the contribution of new physics is not expected. These measurements will set a base for comparison with the measurement where observation of new physics effects is possible. The B^±→DK and B^±→Dπ decays were used with D mesons decays into KK, ππ, K⁰_Sππ, K⁰_SKK or Kπππ. The value of the angle γ = (71.1^+16.6_-15.7)° was obtained using only B^±→DK decay results in the analysis. The image at the left hand side shows the confidence level of the signal as a function of angle γ, for the combination of B^±→DK modes. The peak of the distribution gives the measured central value, and the width gives the error. The best value of the angle γ = 85.1° was obtained with the two corresponding uncertainty regions [61.8,67.8]° and [77.9,92.4]° (at 68% CL) when the B^±→Dπ decays were included in addition.

The 2011 data sample (1.0 fb^-1) was used in this analysis. The precision of the γ angle measurement is already comparable with that achieved by other experiments and the value of this angle is now known to a precision of less than 10 degrees, when this result is averaged with the latest results of other experiments. LHCb physicists are working on analyses of other decay modes that can help improve the precision. It is interesting to note that the data collected by LHCb in 2012 already exceeds the sample from 2011 and by the end of the year the total dataset should have more than tripled.

More details can be found in the LHCb presentation in Cincinnati and in the LHCb Conference Contribution here. Read also the CERN Courier article and the CERN Bulletin article in English and French.

3 October 2012: Matter antimatter asymmetry in three-body charmless B decays becomes more and more interesting. [ A_CP(B^±→π^±π⁺π^-) = +0.120 ± 0.020 ± 0.019 ± 0.007 ]
[ A_CP(B^±→π^±K⁺K^-) = -0.153 ± 0.046 ± 0.019 ± 0.007 ]

Earlier this year, LHCb physicists reported at the ICHEP2012 Conference (see 7 July 2012 (2) news) the first evidence of inclusive CP asymmetry (differences between properties of matter and antimatter) in the charmless three-body B meson decays B^±→K^±π⁺π^- and B^±→K^±K⁺K^- in which the b-quark decays into a u,d or s-quark instead of its dominant decay into a charm c-quark. It was interesting to note that much larger asymmetries were observed in some small special regions, like invariant mass squared of the π⁺π^- pair in the K^±π⁺π^- decay lower than 1 GeV², or of the K⁺K^- pair in the K^±K⁺K^- final state between 1.2 and 2 GeV² (7 July 2012 (2) news).

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These measurements have now, at the 7th International Workshop on the CKM Unitarity Triangle, Cincinnati, Ohio, USA, been complemented with results from the rarer B^±→π^±π⁺π^- and B^±→π^±K⁺K^- decays, finding evidence of even larger CP violation. A remarkable feature of the new LHCb results is that the CP violation effects appear to arise in some special kinematical regions that are not dominated by contributions from narrow resonances. For example, in B^±→π^±K⁺K^- decays a broad feature at low K⁺K^- invariant mass that was previously observed by the BaBar collaboration [PRL 99 (2007) 221801] appears to be present only in B⁺ decays, as shown by the filled triangles distribution in the left figure above, and not in the B^- distribution (open triangles). This points to some interesting hadronic dynamics that could generate the observed direct CP violation. LHCb is now starting detailed studies of these channels, that can also exploit the three times larger data sample available after 2012 running, to further understand these effects. The results of these analysis will also establish whether the observed CP violation is consistent with the Standard Model expectation or has a more exotic origin.

The middle and right images above show the π^±K⁺K^- invariant mass distribution for B^- and B⁺ decays with m²K⁺K^- 1.5 GeV²/c⁴. The difference between properties of matter and antimatter, the CP violation, is clearly seen as a difference between the height of two peaks located at the B mass (at 9σ level for experts).

More details can be found in the CERN Courier article, in the LHCb presentations in Cincinnati and in the CERN seminar here.

25 September 2012: Searching for new physics in rare kaon decays. [ Branching ratio K⁰_S →μμ 9x10^-9 at 90% CL ]

Having previously set the world's most restrictive limits on the dimuon decays of D⁰, B⁰ and B⁰_s mesons, LHCb physicists have turned their attention to the search for similar decays of another member of the particle zoo, the K⁰_S meson. Using the 2011 data sample, LHCb has set a limit on the branching ratio B(K⁰_S→μ⁺μ^-) to be less than 9x10^-9, a factor of 30 improvement over the previous most restrictive limit measured in 1973. [For experts: the limit is at 90% confidence level.] While several techniques used were common with the search for B⁰_s→μ⁺μ^- (see news items of 8 April and 22 July 2011 and 5 March 2012 for more details), the challenge of this measurement lies in the specificity of kaon decays, which are very different from B decays for which LHCb detector was optimized.

The image below shows the invariant mass distribution of selected μ⁺μ^- pairs, candidates for K⁰_S decay. The dashed lines indicate the signal region around the K⁰_S mass, where no significant signal is seen.

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K meson decays into pairs of muons played a very important role in the history of particle physics. There are two types of K mesons: the short-lived, K⁰_S ("K-short") and the long-lived, K⁰_L ("K-long"). The results of branching ratio measurements of the K-long decay into muon pairs in the early 1970s disagreed strongly with the predictions of particle physics theory based on existence of three quarks u, d and s. The calculated branching ratios were of the order of 10^-4 while the experimental limits were about 4 orders of magnitude lower. In order to solve this problem Glashow, Iliopoulos and Maiani proposed the existence of an additional quark, called a charm quark – a 1970s version of a new physics model. In the mechanism they proposed (GIM mechanism) a destructive virtual contribution of this new quark reduced very strongly the K-long decay rate into muon pairs. The discovery of the J/ψ state in November 1974 gave the first evidence for the existence of charm quark and at the same time confirmation of the GIM mechanism.

40 years later LHCb physicists are searching for K-short decays into muon pairs, again on the look-out for new physics. The branching ratio is calculated to be 5x10^-12 within the framework of the Standard Model of particle physics with 6 quarks. Although the new limit 9x10^-9 is still three orders of magnitude above the prediction, it starts to approach the level where new physics effects might begin to appear. Moreover, the data collected by LHCb in 2012 already exceed the sample from 2011 and by the end of the year the total dataset should have more than tripled.

Read more in the CERN Courier article, the CERN Quantum Diaries and LHCb publication.

13 September 2012: First proton-lead ion collisions at LHCb.

During the night, at 1:30 am, LHCb recorded the first proton-lead ion collisions at the LHC. The proton-lead physics data taking is planned to take place in January and February 2013. Today the LHC operational team made tests of collisions in order to prepare the set-up of the LHC collider and the LHC experiments for next year. Note that collisions of protons with lead ions are more difficult than proton-proton or lead-lead collisions. In fact the speed of protons and lead ions is slightly different, even though it is close to the speed of light at LHC. The LHC operators succeeded in making the proton path length inside LHC ring slightly longer than the lead-ion path length in order to compensate for this difference.

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A typical proton-lead collision event at LHCb is shown in the left image above. Note that the lead ions arrive at the LHCb collision point from the right hand side and the protons from the left hand side. The right image shows the invariant mass spectrum of the decay products of Λ and Λ particles indicating excellent prospects for physics analysis of p-lead collision data early next year.

Read also the CERN Courier article.

21 August 2012: LHCb has doubled its recorded luminosity.

The data sample used to obtain the LHCb results presented on this page was obtained using the integrated luminosity of 1.11 fb^-1 recorded by LHCb during the 2010-2011 data taking period. The same inegrated luminosity has just been recorded this year, which corresponds to doubling the total available integrated luminosity. The data sample which can be used for physics analysis has more than doubled in the same time, since it is expected that the cross-sections for bb and cc quark-antiquark pair production have increased by 15% and 12%, respectively, at the increased pp collision energy of 8 TeV this year. LHCb physicists expect to more than triple their data set this year by the end of pp collision run at December, which should allow them to obtain even more interesting physics results.

You can follow live the progress of delivered and recorded luminosity at LHCb, the number of proton-proton collisions visible at LHCb, as well as the number of bb and cc quark pairs produced within the LHCb acceptance at the image above on this page.

3 August 2012: 1 fb^-1 of luminosity has been delivered to LHCb this year.

"This is great achievement considering that it comes about two months earlier than last year. Once more, the excellent performances of the machine, the skill and the commitment of the whole LHC team, made possible this result. We have also the exciting perspective of getting another 1 fb^-1, or more, this year. This increased sample will allow us to push further our knowledge of Standard Model, and find finally where new physics is hiding so well." - said Pierluigi Campana, LHCb spokesperson.

12 July 2012: LHCb film shortlisted by the European Science TV and New Media Festival 2012.

"LHCb - A Beauty Experiment", a short documentary on LHCb, has been shortlisted by the European Science TV and New Media Festival 2012. The festival takes place in association with the ESOF 2012 Science Congress in Dublin at Trinity College on 13-15th July 2012. The goal of the festival is to help writers to develop TV drama that involves science and technology. It is interesting to note that the LHCb film was already shortlisted by the NHK Japan Prize Festival in October 2011. Watch the film on YouTube in different resolutions and with subtitles in 15 different languages.

Screen shots from the film show the LHCb control room on 23 November, 2009 (left) and on 30 March, 2010 (right).

7 July 2012 (1): Measurement of the flavour-specific matter antimatter asymmetry. [ a^s_sl = (-0.24 ± 0.54 ± 0.33)% ]

Physicists from the LHCb experiment have today released results that help to shed light on one of the most significant experimental discrepancies with the Standard Model of particle physics. In 2010, and with an update in 2011, the D0 experiment, analyzing data taken at the proton-antiproton collider Tevatron at Fermilab, reported an interesting observation: that the number of events containing two positively charged muons is lower than the number of events containing two negatively charged muons, see 2010 Fermilab Press Release. The observed difference was close to 1%, measured with almost the full D0 data sample of 9 fb^-1. Like-signed dimuons can be produced from the decay of particles containing the b quark, which can mix between their particle and antiparticle states. A difference between the number of positive and negative dimuons would be an indication of CP violation. The D0 result differs by 3.9σ from the tiny value predicted in the framework of the Standard Model and could indicate the presence of a new physics contribution. This difference can be expressed as an asymmetry, A^b_sl, where the label "b" indicates decay of particles containing b-quarks and "sl" (semileptonic) indicates that the decay involves leptons, in this case muons. The decaying matter (antimatter) B particles are composed of b-antiquarks(quarks) and d- or s-quarks(antiquarks). D0 physicists could not distinguish which type of decaying particles is at the origin of the measured asymmetry, therefore they present the measured asymmetry A^b_sl in the image below as an inclined band across the plane of individual asymmetries in the decays of B_d and B_s mesons, labelled a^d_sl and a^s_sl, respectively. The vertical band shows the measurement of the a^d_sl asymmetry by the BaBar and Belle collaborations working at the Υ(4S) resonance, which is in agreement with the Standard Model calculations shown as the SM point in the image.

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LHCb physicists have presented today at the ICHEP2012 Conference in Melbourne the most precise determination to date of the corresponding asymmetry for the B_s meson, a^s_sl. The LHCb result a^s_sl = (-0.24 ± 0.54 ± 0.33)% is shown as the horizontal blue band in the image above. The B⁰_s semileptonic decays into D^±_sμ^∓ final states were studied to obtain this result. The charmed D^±_s mesons were reconstructed in the φπ^± mode. More details can be found in the LHCb presentation in Melbourne and in the LHCb Conference Contribution here. The LHCb result is consistent with the Standard Model prediction and does not confirm the deviation from the Standard Model reported by the D0 experiment. The D0 experiment previously published a result using D_sμ events, shown as the horizontal yellow band.

The full 2011 LHCb data sample was used to obtain this result. LHCb physicists expect to more than triple their data sample this year.

Read also the CERN Bulletin article in English and French, in the CERN Courier article and the CERN Quantum Diaries.

7 July 2012 (2): Evidence for matter antimatter asymmetry in three-body charmless B decays. [ A_CP(B^±→K^±π⁺π^-) = +0.034 ± 0.009 ± 0.004 ± 0.007 ]
[ A_CP(B^±→K^±K⁺K^-) = -0.046 ± 0.009 ± 0.005 ± 0.007 ]

LHCb physicists have presented today at the ICHEP2012 Conference in Melbourne measurements of differences between properties of matter and antimatter (CP violation asymmetry) in the charmless three-body B meson decays B^±→K^±π⁺π^- and B^±→K^±K⁺K^-. The dominant B meson decays involve a beauty b-quark decay into a charm c-quark. In the rarer charmless decays (without charmed mesons) discussed here the b-quark decays into a u,d or s-quark. LHCb physicists have measured the charge asymmetry A_CP obtained from the difference between negative and positive B event rates. The numerical values of measured asymmetries are shown above. The significance is 2.8σ for the B^±→K^±π⁺π^- decay and 3.7σ for the B^±→K^±K⁺K^- channel. The latter is the first evidence of inclusive CP asymmetry in charmless three-body B^± decays.

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The results presented here are obtained by integrating (summing) the asymmetries over all kinematical variables of particles observed in the decays of B mesons. It is interesting to note that much larger asymmetries are observed in some small special regions, like invariant mass squared of the π⁺π^- pair in the K^±π⁺π^- decay lower than 1 GeV², or of the K⁺K^- pair in the K^±K⁺K^- final state between 1.2 and 2 GeV², as seen in the images above. Note that the value plotted in the figures is the raw asymmetry A^RAW_CP, and does not include all the small corrections that are included in the numerical results. LHCb physicists are planning further study of this intriguing feature.

The full 2011 data sample was used to obtain this result. LHCb physicists expect to more than triple their data sample this year.

More details can be found in the LHCb presentation in Melbourne and in the LHCb Conference Contribution here. Read also the CERN Bulletin article in English and French and the CERN Quantum Diaries.

25 May 2012: Puzzling asymmetries.

The decay of the beauty meson B into an excited K meson K^* and a μ⁺ and μ^- pair is considered as an important channel for new physics search, see 13 March 2012 and 22 July 2011 news. Different distributions and branching fractions have been studied for these B meson decays and compared with the Standard Model predictions. Recently LHCb physicists have studied in addition the differences in the results of measurements of neutral B meson decays into K^*0μ⁺μ^- and charged B⁺ meson decays into K^*+μ⁺μ^-. Theoretical uncertainties of the Standard Model calculations are strongly reduced in these differences. Physicists call these differences "asymmetries" and since this case involves differences between the decays of particles with different charges, it is called an "isospin asymmetry" A_I. The Standard Model calculations predict this isospin asymmetry to be small, as shown with the help of the color line in the left image below, where the prediction is presented as a function of the square of the di-muon invariant mass (q²). The experimentally measured distribution, shown by points with error bars, is consistent with this prediction.

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A surprise came when the physicists made similar analysis by replacing the excited kaon K^* by its ground state K. The negative asymmetry is clearly visible in the right image above (4.4σ different from zero after integration (summation) over the whole q² region). The Standard Model calculation is not yet available for this asymmetry, but is, as in the B_s→K^*μμ mode, expected to be very close to zero.

The images shown above were obtained with the full 2011 data sample. These results have been made possible by the strong and continuous support from the LHC operations team for the LHCb physics program, and the data sample is expected to more than double by the end of this year. In the meantime theorists will analyze this puzzling result in order to establish whether this effect can be accommodated in the framework of the Standard Model, or whether a new physics explanation is required.

Read more in CERN Bulletin article in English and French, in the CERN Courier article and LHCb publication.

16 May 2012: First observation of two excited states of Λ_b.

The quark model, independently proposed by physicists Murray Gell-Mann and George Zweig in 1964 to classify the strongly interacting particles called hadrons, is very successful. In this model baryons are composed of three quarks and mesons are composed of quark-antiquark pairs. The simplest baryon, the proton, which is the nucleus of the hydrogen atom, is composed of three light quarks uud while its neutral partner the neutron is composes of udd quarks. By replacing one of the d quarks by a heavier strange quark s we obtain a Λ particle composed of uds quarks. Furthermore by replacing in the Λ the s quark by a charm quark c or a beauty quark b we obtain a Λ_c or a Λ_b particle.

The three quarks forming the Λ, Λ_c and Λ_b are in their lowest quantum mechanical state. Like electrons in atoms quarks can form excited states with different values of angular momentum and quark spin orientation. These excited states were previously observed for the Λ and Λ_c particles. They were, however, never observed for the Λ_b particle.

The LHCb collaboration has made first observations of two excited states of Λ_b.

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The Λ_b excited states have been reconstructed in three steps. In the first step the Λ_c⁺ particles were reconstructed through their decay into a proton p, a negative K^- meson and a positive π⁺ meson. In the second step the Λ_c particles were combined with negative π^- mesons in order to form the Λ_b particles. The Λ_b signal is clearly seen as the enhancement in the left image above showing the Λ_c⁺π^- invariant mass spectrum. Finally the Λ_b particles have been combined with a pair of opposite sign pions π⁺π^-. In the right image above two enhancements are clearly seen corresponding to the two Λ_b excited states with masses of 5912 and 5920 MeV, about 6 times the proton mass.

LHCb physicists have observed about 16 Λ_b(5912)→Λ_bπ⁺π^- decays (5.2σ significance) and about 50 Λ_b(5920)→Λ_bπ⁺π^- decays (10.2σ) among about 60 million million (6*10¹³) pp collisions seen by the LHCb detector at LHC during the 2011 data taking period.

Read more in CERN Bulletin article in English and French, in the CERN Courier article and LHCb publication.

27 April 2012: The rarest B decay ever observed.

The LHCb collaboration has made the first observation of the decay B⁺ → π⁺μ⁺μ^-. With a branching ratio of about 2 per 100 million decays, this is the rarest decay of a B hadron ever observed, see CERN Courier article.

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The image shows the invariant-mass distribution of selected π⁺μ⁺μ^- combinations, showing the clear peak corresponding to B⁺ decays (green long-dash). Also shown are the components in the fit from partially reconstructed decays (red dotted) and misidentified K⁺μ⁺μ^- (black dashed) and the total (blue solid line). Candidates for which the μ⁺μ^- pair is consistent with coming from a J/ψ or ψ(2S) decay have been excluded.

6 April 2012: LHCb looks forward to electroweak physics.

Measurements of the production of W and Z bosons, see 10 June and 22 July 2010 news, allowed LHCb physicists to explore electroweak physics, see CERN Courier article.

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The two images above show the complementarity of measurements made by different LHC experiments. The left image shows the x-Q² kinematic region explored by the LHCb experiment (pink), compared with ATLAS and CMS (light green), as well as previous experiments. The right image shows LHCb's measurement of W charge asymmetry, shown as a function of lepton pseudorapidity, compared with theoretical predictions, and with results from the ATLAS and CMS experiments. More details and explanations can be found in the CERN Courier article.

30 March 2012: LHCb strongly squeezes SUSY parameter space.

Results presented by the LHCb Collaboration at the Rencontres de Moriond EW and QCD conferences allowed theorists to squeeze strongly the parameters of supersymmetric extensions of the Standard Model (SUSY), the most popular new physics model. The simplest version of this model, called the Minimal Supersymmetric Standard Model (MSSM), predicted the frequencies with which B_s and B_d mesons decay into pairs of oppositely charged muons to have values significantly different from the Standard Model (SM) prediction. This is shown in the left image below, which was presented by David Straub (SNS and INFN, Pisa) at the Moriond EW conference. The predictions for both frequencies (branching ratios BR) depend on different parameters of the MSSM and cover nearly all of the left image surface. The LHCb results, see 5 March 2012 news, limit the predictions that are still allowed to a small region around the SM expected value. It is interesting to note that certain combinations of MSSM parameters allow lower BR values than those predicted by the SM. The LHCb measurements of the parameter φ_s, which sets the scale for the difference between properties of matter and antimatter for the strange beauty B_s mesons, see 5 March 2012 news, also strongly limits the SUSY parameter space that is still allowed, as shown by the vertical lines on the right image below.

SUSY contributions to observables that can be measured in experiments depend, in general, on more than 100 free parameters. Therefore in order to be able to analyse experimental data physicists are using a simplified model, the Constrained MSSM (CMSSM), with 5 parameters m₀, m_1/2, A₀, tan β and μ/|μ|. Nazila Mahmoudi (Clermont-Ferrand and CERN) presented the left image below at the Moriond QCD conference. The parameter space below and to the left of the red line is excluded by the results of searches for direct production of SUSY particles at the CMS experiment, while the large yellow region shows the parameter space excluded by the analysis of B_s →μμ decays at LHCb. The image is made for a relatively high value of the parameter tan β=50. The LHCb exclusion region is smaller at lower values of tan β as can be seen in the right image below, made with tan β=35. The green regions on both images are still not excluded by LHCb's measurements.

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The images above illustrate that direct and indirect SUSY searches at LHC are complementary and show that the allowed parameter space is characterized by low tan β and high m₀ and m_1/2 within the CMSSM. LHCb's results have ruled out large deviations in several observables, but now LHCb physicists can start to probe the most interesting parameter region of new physics.

13 March 2012 (1): B⁰_d →K^*μμ, a first measurement of the zero-crossing point. [ Zero crossing point B⁰_d →K^*μμ = 4.9^+1.1_-1.3 GeV² ]

LHCb physicists have continued their search for physics beyond the Standard Model using the B⁰_d decay into a K^* meson (an excited kaon), and a μ⁺ and μ^-. New physics contributions can change various distributions that describe the decay process. For example, the number of decays as a function of the square of the di-muon invariant mass (q²) and the di-muon forward-backward asymmetry (A_FB) can both be affected in many new physics scenarios. The variable A_FB indicates whether more or fewer muons of one sign are observed in the same direction as the K^* than opposite to it. The distribution of A_FB in function of q² is shown below. The results have been announced today at the Rencontres de Moriond QCD conference.

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The point at which the A_FB distribution is crossing zero is well predicted within the Standard Model, and any deviation could indicate a possible contribution from new physics. LHCb physicists have already presented at the 2011 summer conferences a first indication that the asymmetry is changing sign. Today, using three times higher statistics, they presented the first measurement of the zero-crossing point of 4.9^+1.1_-1.3 GeV². This value, showed by the hatched vertical region in the Figure, is in agreement with the Standard Model prediction showed by the colored line.

The LHCb Collaboration aims to more than double its data set this year. With the new data, the zero-crossing point will be measured with higher precision, and a possible difference with the Standards Model prediction may be discovered.

Read more in the LHCb staff page.

13 March 2012 (2): A first measurement of the CP asymmetry in the decay B⁰_s →K⁺K^-.

In a fascinating world of quantum mechanics the strange beauty particle (matter) B⁰_s turns into its antimatter partner about 3 million million times per second (3*10¹²), see 15 March 2011 news. Therefore CP violation effects, which are differences between the properties of matter and antimatter, can appear as variations with decay time. A name "time-dependent CP violation" is therefore used.

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The left figure shows the CP asymmetry in the B⁰_d decay into π⁺π^- pair and the right figure shows the asymmetry in the B⁰_s decay into K⁺K^- pair. It can be clearly seen, by comparing time scales of both figures, that the B⁰_d mesons oscillate much slower than the B⁰_s mesons. A new analysis from LHCb has measured the CP asymmetry separated into two components: the difference in the decay rate of B meson and anti-B meson (physicists call it "direct CP violation") and the quantum-mechanical phase difference between the B meson and anti-B meson decay ("mixing-induced"). These two components appear as a cosine-like and as a sine-like oscillation in the asymmetry plots above.

The results, announced today at the Rencontres de Moriond QCD conference, for the B⁰_d → π⁺π^- decay are A^dir_π⁺π^- = 0.11 ± 0.21 ± 0.03 and A^mix_π⁺π^- = -0.56 ± 0.17 ± 0.03, meaning the oscillation appears to come mainly from the sine-like component. These are the first measurements of these quantities at a hadron collider, and are consistent with previously published results from other experiments. The results for the B⁰_s → K⁺K^- decay - which are measured for the first time ever - are A^dir_K⁺K^- = 0.02 ± 0.18 ± 0.04 and A^mix_K⁺K^- = 0.17 ± 0.18 ± 0.05.

About 2/3 of data taken in 2011 were used in this analysis. The LHCb Collaboration aims to analyse three times more data by the end of this year. This will allow to see if CP violation occurs in the B⁰_s → K⁺K^- decay, and see if the amounts of direct and mixing-induced CP violation are as expected by the Standard Model.

Read more in the LHCb staff page.

5 March 2012 (1): Search for New Physics, an important milestone [ Branching ratio B⁰_s →μμ 4.5x10^-9 at 95% CL ]

The LHCb collaboration has announced today at the Rencontres de Moriond EW conference one of the most hotly anticipated results from the LHC. LHCb has shown that the frequency with which a B_s meson decays into a pair of oppositely charged muons is not larger than 4.5 times out of one billion decays. Theorists have calculated that, in the Standard Model, this decay should occur about 3 times in every billion, but that if new particles predicted by theories such as supersymmetry exist, the decay could occur much more often (see news items of 8 April and 22 July 2011 for more details). The new results represent a milestone in the search for "new physics" beyond the Standard Model.

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The left figure shows the mass calculated from the two muons for events that survive all selection requirements. The blue area shows the shape of random combinations of muons, while the red area shows the shape expected for real B_s decays (with the hatched area indicating the uncertainty on the sum of the two contributions). The data are seen to be consistent with a small excess over the background-only hypothesis. This excess is slightly less than, but consistent with, the Standard Model expectation, as shown in the right figure. The blue points show how likely the data distribution is for a given rate of B_s decay. The black dashed line shows the expected shape of the curve for the Standard Model rate, with the green band indicating the uncertainty. The horizontal lines allow to set limits with different degrees of certainty: for experts, the solid red line gives the 95% C.L.

Measuring the rate of this B_s decay has been a major goal of particle physics experiments in the past decade, with the limit on its decay rate being gradually improved by CDF, D0, LHCb and CMS experiments. The latest results by LHCb set the tightest limits to date. More data is needed to finally discover if the decay occurs at a rate above, below, or at that predicted by the Standard Model. LHCb aims to more than double the size of its data set in 2012, which could be enough to finally answer this question.

Read more in CERN Press Release CERN Bulletin article, in the CERN Quantum Diaries blog and also in the LHCb staff page.

5 March 2012 (2): Heavier strange-beauty lives longer and improved φ_s measurements. [ φ_s = -0.002 ± 0.083 ± 0.027 rad]

LHCb physicists have reported today at the Rencontres de Moriond EW conference important progress in measurement of the difference between properties of matter and antimatter for the strange beauty B_s mesons. The new results improve on those presented at the summer 2011 conferences. The size of this difference is controlled by the parameter φ_s, which is predicted to be small in the Standard Model. However, effects of new particles not predicted by the Standard Model can make the measured value much larger. The progress is shown in the image below: the remaining allowed region is shown in yellow and compared to the previous results from LHCb in blue, and from CDF and D0 experiments in green and red, respectively.

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In the mysterious world of quantum mechanics the strange-beauty meson B_s matter antimatter system is alternatively described as a heavy and light mass B_s meson system. The mass difference Δm_s determinates the matter antimatter oscillation frequency, see 15 march 2011 news. The difference of their lifetimes was measured together with the value of φ_s. Since it was not previously know if the heavier or lighter B_s mesons live longer, LHCb physicists have obtained two possible values for φ_s corresponding to two blue regions in the image above. (The axis label ΔΓ_s corresponds to the difference of inverse lifetimes between the heavy and light B_s meson, physicists call it width (Γ) difference.) Recently LHCb physicists have succeeded to measure that the heavier strange-beauty B_s mesons live longer and in this way eliminated one of two blue regions in the image. Sophisticated quantum mechanical interference effects were used in this measurement. Experts can read details of the analysis in the published article.

The full sample of data collected in 2011, three times larger than that used in summer 2011, was used to obtain the result
φ_s = -0.001 ± 0.101 ± 0.027 rad using B_s decays into into a J/ψ meson and a φ meson; combining it with the measurement of the B_s decay into J/ψ and f₀(980) LHCb physicists have obtained the final result φ_s = -0.002 ± 0.083 ± 0.027 rad.

2012 data taking period will start soon. Search for new physics in the small yellow region will continue.

Read more in CERN Press Release CERN Bulletin article, in the CERN Courier article, in the CERN Quantum Diaries blog and also in the LHCb staff page.

5 March 2012 (3): First evidence for CP violation in the decays of B_s mesons.

Measurement of CP violation, which describes differences between the properties of matter and antimatter, is a very important goal of LHCb. CP violation is well established in the K⁰ and B⁰ meson systems. Recent results from the LHCb collaboration have also provided evidence for CP violation in the decays of D⁰ mesons. Consequently, there now remains only one neutral heavy meson system, the B⁰_s, where CP violation has not been seen.

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The LHCb collaboration has recently submitted for publication the first evidence for CP violation in the decays of B⁰_s mesons. The paper (available here for experts), submitted to Physical Review Letters, confirms the preliminary results shown at the EPS-HEP meeting in Grenoble, see 22 July 2011 news. The B meson decays into K and π mesons were studied. The decay into a positive K meson red track and a negative π green track is show in the event display above. The four plots on the right hand side show the Kπ invariant mass distribution divided into different components as shown by the legend in the top-right figure. The different charge combination of K and π indicates if the decaying B particle is a matter or an antimatter particle. The two upper plots show that the decay rates of B⁰_d mesons are different, as known from previous experiments. The lower two plots show that the difference is also visible for the B⁰_s mesons. The measured CP asymmetry for the B⁰_d mesons A_CP = -0.088 ± 0.011 ± 0.008 constitutes the most precise measurement available to date and the first observation (6σ) at a hadron collider. The measured CP asymmetry for the B⁰_s mesons A_CP = 0.27 ± 0.08 ± 0.02 is the first evidence (3.3σ) for CP violation in the decays of these mesons.

LHCb physicists have used 1/3 of data collected in 2011 in this analysis and expect to have ten times more data by the end of this year.

Read more in CERN Press Release CERN Bulletin article, in the CERN Quantum Diaries blog and also in the LHCb staff page.

1 December 2011: LHCb looks to the future.

After a very succesful 2011 data taking period the LHCb Collaboration is preparing next year's operation. The first 2012 collisions should be observed in April. At the same time LHCb physicists are also actively working on the longer term future in which data will be taken at a much higher rate, see CERN Courier article.

14 November 2011: CP violation in charm decays. [ ΔA_CP = (-0.82 ± 0.21 ± 0.11)% ]

The LHCb Collaboration has presented today at the Hadron Collider Particle Symposium in Paris possible first evidence for CP violation, the difference between behaviour of matter (particles) and antimatter (antiparticles), in charm decays. The study of CP violation in both charm and beauty particle decays is central to the LHCb physics programme. In the Standard Model CP violation is expected to be very small in the charm sector, whereas new physics effects could generate enhancements.

In this new analysis the LHCb physicists have used data collected in the first half of the 2011 run to study the differences in decay rates of neutral D meson particles composed of a charm quark c bound with an up antiquark (u) and D meson antiparticles (D) composed of a charm antiquark (c) bound with an up quark (u). The decays of D^*+ mesons into D mesons and π⁺, and D^*- mesons into D mesons and π^- were used to select the D and D mesons. In the next step of the analysis the difference (asymmetry A_CP) between the decay rates of D and D mesons into K⁺K^– pairs as well as into π⁺π^- pairs was measured. By determining the difference, ΔA_CP, in CP asymmetries for the K+K- and π⁺π^- decays, the analysis strongly suppresses possible measurement biases which could arise through effects related to particle production, selection etc. The following preliminary result is obtained:

ΔA_CP = (-0.82 ± 0.21 (stat.) ± 0.11 (sys.) )% [ 3.5 sigma significance for experts ]

A very interesting period now begins. LHCb physicists are analysing the remainder of the data collected in 2011. If the result is confirmed theoretical work will be required to establish whether this effect can be accommodated in the framework of the Standard Model, or whether a new physics explanation is required.

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The figures show the invariant mass distribution of the K^-K⁺ (around 1.4 million) and π^-π⁺ (around 0.4 million) pairs. The distributions are centered at the D meson mass of 1865 MeV.

Read more in CERN Bulletin article in English and French, in the CERN Courier, in the CERN Quantum Diaries blog in English and French and also in the LHCb staff page.

30 October 2011: End of 2011 proton-proton collision data taking period.

The 2011 proton-proton collision data taking period has ended today. LHC collider and LHCb experiment have been working extremaly well. LHCb has recorded all in all an impressive 1.1 fb^-1 out of a 1.22 fb^-1 delivered at 3.5 TeV.

“We’ve got from the LHC the amount of data we dreamt of at the beginning of the year and our results are putting the Standard Model of particle physics through a very tough test ” said LHCb Spokesperson Pierluigi Campana. “So far, it has come through with flying colours, but thanks to the great performance of the LHC, we are reaching levels of sensitivity where we can see beyond the Standard Model. The researchers, especially the young ones, are experiencing great excitement, looking forward to new physics.”

Read more in CERN Press release in English and French.

3 October 2011: 1 fb^-1 of luminosity has been delivered to LHCb.

This is a very important milestone for LHCb which will allow LHCb physicists to reach an unprecedented accuracy in most of the core physics processes that are under study.

Read more in CERN Bulletin article in English and French.

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The left hand image shows Mike Lamont (Operations Group Leader), Pierluigi Campana (LHCb Spokesperson), Steve Myers (Director for Accelerators and Technology), and Paul Collier (Head of the Beams Department) celebrate the LHCb milestone. The right hand image shows the zoom on the computer screen located above them and showing the LHC screen congratulating LHCb for its new record.

3 October 2011: LHCb film shortlisted by Japan Prize Festival

"LHCb - A Beauty Experiment", a short documentary on LHCb, has been shortlisted by the NHK Japan Prize Festival. Every year, the festival awards the very best in global educational media. Watch the film at YouTube in different resolutions and with subtitles.

27 August 2011: φ_s: different properties of matter and antimatter for B_s mesons [ φ_s = 0.03 ± 0.16 ± 0.07 ]

LHCb physicists have presented today the most precise measurement of φ_s (the B_s mixing phase, for experts) at the Lepton Photon conference in Mumbai (India). The value of φ_s is precisely predicted in the Standard Model and sets the scale for the difference between properties of matter and antimatter for B_s mesons, known to physicists as CP violation. The predicted value is small and therefore the effects of new physics could change its value significantly - see the analogy in the 8 April news.

The decay of the strange-beauty particle B⁰_s, composed of a beauty antiquark (b) bound with a strange quark (s), into a J/ψ meson and a φ meson was used for this measurement. The J/ψ meson decays in turn into a μ⁺μ^- pair, and the φ decays to K⁺K^– pair. In order to make this difficult measurement LHCb physicists had to analyse the B_s decay particles in 3 dimensions as well as to measure precisely the fast oscillations of strange beauty - see 15 March news.

The "artist's view" below shows the result of the φ_s measurement in a plane together with the correlated measurement of another value, ΔΓ_s. The results of the measurement favour two regions, one of which is located around φ_s = -0.036 ± 0.002 rad, the Standard Model prediction. The LHCb measurement is in agreement with the Standard Model prediction but the shaded region representing the LHCb result indicates that there is still room for a new physics contribution. The hints for a larger contribution of new physics suggested by the CDF and D0 experiments at Fermilab, also shown in the figure, are not confirmed.

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In February the LHCb Collaboration made first observation of the B_s decay into J/ψ f₀(980) - see 27 February news. This decay contributed to the φ_s measurement.

Using both B_s decays LHCb physicists have obtained the value φ_s = 0.03 ± 0.16 ± 0.07.

Read also CERN Press Release in English and French.

Read also CERN Bulletin article in English and French.

Read also CERN Courier article.

22 July 2011 (1): Hunting for New Physics continues [ Branching ratio B⁰_s →μμ 1.2x10^-8 at 90% CL and 1.5x10^-8 at 95% CL ]

LHCb physicists continue their search for new physics, see 8 April 2011 news for introduction. They have presented updated results during the International Europhysics Conference on High Energy Physics (EPS-HEP) at Grenoble this week. The LHCb physicists have succeeded in setting the limit for an enhanced decay rate of the strange beauty particle B⁰_s, composed of a beauty antiquark (b) bound with a strange quark s, into a μ⁺ and μ^- pair, as low as about 4 times the rate calculated within the Standard Model (limits 1.2x10^-8 at 90% CL and 1.5x10^-8 at 95% CL for experts). This result was obtained from the analysis of about 8 times more data than in the previous analysis.

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The computer reconstructed images above show the most significant event compatible with the strange beauty B⁰_s decay into muon pair seen as a pair of purple tracks traversing the whole detector. The right hand image shows a close-up around the proton-proton interaction point from which many tracks originate. The B⁰_s decays about 1 cm from the proton-proton collision point into two muons (purple tracks). The invariant mass of muon pair corresponds to the B⁰_s mass. The number of observed B⁰_s candidates is slightly above the background predictions and compatible with the expected signal predicted by the Standard Model (SM) theory. This analysis is much more sensitive than previous experiments, and although large deviations (by more than a factor 4) from the SM are excluded, there is still plenty of room for new physics contributions. LHCb physicists are expecting to be able to analyse about three times as many events by the end of 2011, and about ten times more by the end of 2012, to give a final answer for the possibility of a new physics contribution to this interesting rare decay.

22 July 2011 (2): Hunting more for New Physics

LHCb Physicists have also presented results of their search for new physics using the B⁰_d (composed of a beauty antiquark (b) bound with a down quark d) decay into an excited K meson, K^*, and a μ⁺ and μ^-. The partial rate as a function of the square of the di-muon invariant mass (q²) and the di-muon forward-backward asymmetry (A_FB) can both be affected in many new physics scenarios. The variable A_FB indicates whether more or fewer muons of one sign are observed in the forward direction than in the backward direction. The distribution of A_FB in function of q² is shown below.

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The LHCb precision measurements shown in black are in agreement with the Standard Model (SM) theory and indicate for the first time that the asymmetry is changing sign as predicted by the SM. The results of measurements by other experiments are presented in the figure on the right hand side.

22 July 2011 (3): Different properties of matter and antimatter

An important part of the LHCb physics programme is reserved for studying differences between the properties of matter and antimatter (CP violation for experts). At the EPS-HEP meeting in Grenoble this week the LHCb physicists have presented distributions in which these differences can be clearly seen.

click the image for higher resolution

The blue line follows the measured points. After background subtraction (slightly sloping black line from left to right) the Kπ invariant mass distribution is divided into different components shown in different colours. The different charge combination of K and π indicates if the decaying B particle is a matter or an antimatter particle. The red curve in the upper plots shows the decay rate of B⁰_d (matter, left plot) and anti-B⁰_d (antimatter, right plot). The horizontal red dotted line helps to show that the decay rates of matter and antimatter B particles to Kπ mesons are different. The green distributions in lower plots show that the difference between matter and antimatter decay rate is also observed in B⁰_s decays to Kπ.

1 June 2011: Pierluigi Campana - new spokesperson for the LHCb collaboration

Pierluigi Campana from the Istituto Nazionale di Fisica Nucleare in Frascati, begins his 3-year tenure as LHCb spokesperson this June. He replaces Andrei Golutvin, from Imperial College London and Russia’s Institute for Theoretical and Experimental Physics. As the new voice for the collaboration, Campana will lead the experiment through what should prove to be a very exciting phase.

Read more details at the LHCb Collaboration Web page and in the CERN Bulletin article in English and in French.

8 April 2011: Hunting for New Physics

Quantum mechanics allows energy non-conservation during a very short time, typical in particle collisions. This opens up the possibility to study signal for the existence of particles for which there is insufficient energy to produce them directly. This feature is used by LHCb physicists to search for heavy particles expected in new physics models - models that describe physics outside the Standard Model (SM) of particle physics. Since the effects of new physics are expected to be very small, LHCb physicists are looking for modifications of the properties of very rare SM processes which can be calculated with high precision. The rare decay of the strange beauty particle B⁰_s, composed of a beauty antiquark (b) bound with a strange quark s, into a μ⁺ and μ^- pair is an excellent candidate. Only one out of 3*10⁹ B⁰_s mesons should decay into a μ⁺μ^- pair according to precise SM calcutions. This rate could be higher if new physics particles, such as those in models with an extended Higgs sector, for example, were to influence this decay.

LHCb physicists have not seen these decays in the data taken during the 2010 run and were able to set a limit which is about 19 times higher than the SM prediction, that is, the limit close to the one set by the Fermilab experiments CDF and D0 after many years of data taking. The figure below was used to set this limit at different levels of statistical probability.

click in the image to get it in higher resolution

LHCb physicists expect to observe the B⁰_s mesons decay into μ⁺μ^- pairs with the rate calculated within the SM using data taken this year and in 2012. An earlier observation of a higher rate will give interestng evidence for new physics.

Read more details in the CERN Courier article.

25 March 2011: LHCb movie

LHCb live and LHCb physics are presented in a short movie (about 15 min) "LHCb - the beauty experiment", available at YouTube and CDS. The YouTube version allows you to choose subtitle language (cc just below the movie window), as well as the image resolution including High Definition version 1080p.

15 March 2011: Oscillating Strange Beauty
or how matter turns into antimatter and back

Using data collected in proton–proton collisions at the LHC at a centre-of-mass energy of 7 TeV LHCb has observed a fascinating feature of quantum mechanics. The strange beauty particle (matter) B⁰_s composed of a beauty antiquark (b) bound with a strange quark s turns into its antimatter partner composed of a b quark and an s antiquark (s) about 3 million million times per second (3*10¹²). The B⁰_s particles have been identified through their decay into strange charm D_s particles (composed of a charm quark c bound with a strange s antiquark) and one or three πs. Of course, LHCb observes B⁰_s particles and antiparticles only during their short lifetime in which they travel about 1 cm in the LHCb detector.

The plot shows the observation of these oscillations when all data have been folded into one oscillation period. The variable A_mix is proportional to the difference between the number of events in which the produced matter(antimatter) B⁰_s particle had the same identity during its decay, and the number of events in which it had not, as a function of its lifetime.

click in the image to get it in higher resolution

The B⁰_s oscillations were observed for the first time by the Fermilab experiments CDF and D0 in 2006, see Press Release article. The oscillation parameters measured by the LHCb Collaboration show agreement with those measured at Fermilab.

27 February 2011: LHCb makes first observations of interesting B⁰_s decays

Using data collected in proton–proton collisions at the LHC at a centre-of-mass energy of 7 TeV, the LHCb experiment has observed two new rare decay modes of B⁰_s mesons for the first time. The decay B⁰_s → J/ψ f₀(980) will be important for studying differences between properties of matter and anti-matter (CP violation for experts) in the B⁰_s system, while the decay B⁰_s → D^*–_s2Xμ⁺ν will be valuable for testing predictions of strong interaction (QCD) theory.

The first new decay mode observed is of the decay B⁰_s → J/ψ f₀(980). The B⁰_s consists of a b antiquark (b) bound with an s quark, and can decay to a J/ψ (cc) together with an ss state, which can be a φ or, more rarely, an f₀. While the φ decays to K⁺K^–, the f₀ decays to π⁺π^–. The figure shows the enhancement in the π⁺π^– invariant mass distribution in the region of 980 MeV indicating an observation of f₀(980).

click in the image to get it in higher resolution

The observation of these two new decay modes demonstrates that the LHCb experiment is already competitive in the field of heavy flavour physics. Great progress is expected with the larger data sample due from the coming run, with the potential to constrain, or even observe, new physics.

Read more details in the CERN Courier article, which also includes description of the second decay.

Read more details in the National Science Foundation article.

27 October 2010: Exotic mesons

In 1964 Murray Gell-Mann and George Zweig proposed the quark model (QM) in which mesons, like π mesons, are formed from quark and anti-quark pairs and baryons (like protons) from three quarks. The model is very succesful, but recently particles which could not be classified in this model have been discovered. LHCb has observed one of these exotic state candidates called X(3872) using its decay into a J/ψ meson (see 6 September news) and a π⁺ π^- pair. The J/ψ decays in turn into a μ⁺ and μ^- pair. The invariant mass of J/ψ π⁺ π^- is shown in the figure below. The left enhancement at the mass of 3686 GeV is consistent with the QM bound state ψ' of charm and anti-charm quarks, the right one at the mass of 3872 GeV has properties that are very difficult to reconcile with the Gell-Mann Zweig QM. Possible explanations include a meson-meson molecule (DD* for experts) or multi quark anti-quark system (diquark-diantiquark tetraquark meson for experts).

this plot was made using all data taken in 2010, click in the image to get it in higher resolution, click here to see original plot.

The particle with the mass of 3872 Gev was first observed by the BELLE collaboration in 2003 and is called X(3872). The observation of this particle at this early stage of data taking by LHCb confirms the excellent performance of the LHCb detector and data analysis.

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13 October 2010: LHCb control room in action

LHCb physicists discussing data taking.

click in the image to see other photos.

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24 September 2010: Young scientists at LHCb

The young scientists took part in the LHCb shifts during the European researchers’ night on Friday 24 September.

click in the image to see other photos.

Read CERN Bulletin article in English and French, as well as the one in the Symmetry Breaking.

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6 September 2010: Beautiful atoms

The LHCb has observed beautiful atoms. The atoms are bound states of the beauty quark and anti-beauty quark. The atoms are bound by the strong force, the force which also binds quarks inside proton. The beautiful atom is 10 times heavier than the proton (yes, we can create mass from energy using famous Einstein formulae E=mc²), has a size sligtly smaller than the size of the proton but about 100 000 times smaller than the size of the hydrogen atom which is composed of a proton and an electron and is bound by the electromagnetic force. Just like ordinary atoms beauty and anti-beauty quarks form different quantum states with different angular momenta and different spin orientations (see figure below right). Only the states marked 1S, 2S and 3S are observed at LHCb by detecting their decay into a μ⁺ and μ^- pair (left).

The figures above show the invariant mass of μ⁺ and μ^- particles (left) and the schematic view of the beautifull atom quantum states (right), click in images to get them in higher resolution. The invariant mass plot was made using all data taken in 2010, click here to see original plot.

The beauty-anti-beauty atom, called "Upsilon" was discovered in 1977 at Fermilab near Chicago.

The states 1S, 2S and 3S do not decay into Beauty Particles since their mass is lower than the sum of masses of Beauty and anti-Beauty particles (BB threshold in the figure). On the other hand the state 4S does decay. This feature is used by the experiments BABAR and BELLE producing the 4S state at e⁺e^- colliders as a source of Beauty and anti-Beauty particles.

The charm and anti_charm quarks form two bound atom states 1S and 2S called J/ψ and ψ' observed at LHCb through their decay into a μ⁺ and μ^- pair (left) and a e⁺ and e^- pair (right).

click in images to get them in higher resolution.

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22 July 2010: From a B to Z, LHCb explores the particle alphabet

LHCb has unveiled pictures of a Z boson inside the experiment. This boson is one of the best understood of all particle species. It shows us how the forces of electricity, magnetism and radiation are connected inside the Standard Model, our theory of particle physics. Measurements of how often we see Z bosons inside LHCb will provide a sensitive test of how well our theory describes this particle at the record breaking energies of the LHC.

click in images to get them in higher resolution

In this picture the Z boson has decayed immediately to two muons μ, shown by the thick white lines which point to the green muon chamber hits in the outer circle of the Eolas display (described in 10 June 2010 News). Not much else happens inside LHCb when a Z is at work – only a few other particles are visible – and this makes it an easy particle to find. We’re looking forward to collecting more of them now, and really testing how well the Standard Model performs for us.

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12 July 2010: LHCb is younger!

The average age of the LHCb Collaboration members has strongly decreased after arrival of the LHCb summer students seen below in front of the Globe of Science and Innovation. The Summer Students follow the lectures given at the first floor of the Globe in the morning. During the breaks they can visit the new CERN exhibition "Universe of Particles" located on the ground floor and the rest of the time they make an important contribution to the LHCb data taking and data analysis.

click in image to get it in higher resolution

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10 June 2010: The W boson in action

LHCb has taken its first snapshots of the W boson in action. This particle conveys the weak force, which makes certain forms of radioactivity possible. It is shown here having decayed to a muon μ (shown as a straight white line, pointing to the filled green muon detector hit circles in the 2D picture, and as a red line pointing to blue muon hits in the 3D picture), which we see, and a neutrino ν, which we don't, with very little else around it.

click in images to get them in higher resolution

Eolas (gaelic for 'knowledge') is a 2D view of a collision inside LHCb. It is a transmogrified view, chosen to illustrate where particles deposit energy as they fly outwards from the collision point. The radius represents flight through the detector along the beam direction - through the tracking detectors, then the first muon chamber, then the electromagnetic and hadronic calorimeters, and finally the last four muon chambers. The φ angle represents the angle in the x,y direction perpendicular to the beam. Information is colour coded. Particle tracks are shown by the dashed lines. The transverse momentum of the particle is shown by the solid white long along this path - the higher this is, the longer the solid white bar is. Yellow bars show energy deposited in the electromagnetic calorimeter, cyan energy deposited in the hadronic calorimeter. Deposits in muon chambers are illustrated by green circles. If these are filled, they are associated with a particle track passing through them.

We will use samples of W bosons to test our theory of particle physics, the Standard Model, to high precision. This is exciting because we don't know yet if our theory holds at LHC energies - if it doesn't, if there are new particles to find in nature, we'll see W bosons behaving in a way we don't expect. With these first snapshots taken, we're on our way to finding out.

The W boson was discovered in 1983 at CERN by the UA1 and UA2 experiments giving the Nobel Prize to Carlo Rubbia and Simon van der Meer.

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7 May 2010: Strange Beauty and Charm

LHCb has reconstructed an event having all characteristics of a Strange Beauty Particle decay! A computer view of this event is shown below. The Strange Beauty Particle (called B_s) is composed of an anti-quark b (b is for beauty) and a quark s (s is for strange). It is produced by the collision of two 3.5 TeV protons from the LHC at a location marked as "PV" (Primary Vertex), together with many other particles (not shown). The B_s decays after travelling about 1.5 mm into three particles called μ^-, D_s⁺ and neutrino ν at a place marked "SV" (Secondary Vertex). The ν is not detected since it can even traverse the whole Earth without any interaction. The Charm Particle D_s⁺ is composed of a c quark (c is for charm) and anti-quark s. The D_s⁺ particle decays in turn after travelling 6.5 mm into three long lived particles K⁺, K^- and π⁺ in a place called "TV" (Tertiary Vertex). The K⁺, K^- and π⁺ are traversing the LHCb detector where the tracking system is used to reconstruct their trajectories with such a very high precision that it is clear that the particles come from three different places called vertices.

click in image to get it in higher resolution

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21 April 2010: First reconstructed Beauty Particle

LHCb has reconstructed its first Beauty Particle! You can see below a computer view of this event in two projections (images on the left hand side). The Beauty Particle (called B⁺) is composed of an anti-quark b (that has a very short lifetime of 1.5 thousandth of a nanosecond!) and a quark u. It is produced by the collision of two very high energy protons from the LHC at a location marked as "Primary vertex", together with many other particles (shown in black). The B⁺ decays after travelling about 2mm into two particles (called J/ψ and K⁺) at a place marked "B decay vertex". The J/ψ particle decays in turn immediately into two long lived particles called μ⁺ and μ^-. The μ⁺ , μ^- and K⁺ are traversing the LHCb detector where the tracking system is used to reconstruct their trajectories with such a very high precision, that it is clear they do not come from the primary vertex. The fact that the reconstructed tracks do not cross exactly in two points reflects experimental precision of computer reconstruction. The real particle tracks originate at the two vertices. The images on the right hand side show the same event when the tracks from the "Primary vertex" are forced to come from the "Primary vertex".

click in images to get them in higher resolution

The LHCb physicists have collected about 10 million proton-proton collisions in order to find this first Beauty Particle. The reconstruction of each event is not easy, there are about 100 particle tracks reconstructed in this event, see full event display below.

More details: LHCb physicists have calculated invariant mass of μ⁺ and μ^- particles from the "B decay vertex" and found that it correspond to the J/ψ mass, see below invariant mass distribution of all μ⁺ and μ^- pairs with the peak corresponding to the J/ψ decays. The reconstructed invariant mass of J/ψ and K⁺ is 5.32 GeV, in agreement with to the known B⁺ mass, 5.5 times higher than the colliding proton mass but 650 times smaller than the colliding proton energy (yes, we can create mass from energy using famous Einstein formulae E=mc²).

see comments in articles: NewScientist internet, magazine and ZDNet.

First

30.3.2010

Both proton beams made a full turn of LHC on Feb. 28th. A new period of great measurements with LHCb has started again and will continue for 18-24 months. On March 18th both beams have been accelerated to 3.5 TeV,

3.5x3.5 TeV

30.3.2010

see TV footage with preparation for collisions and photo above taken just after the first collisions at 12:59 on March 30 (click on picture to get it in higher resolution).

more pictures can be found here, here, and here.

proton-proton collisions

30.3.2010

Our first 3.5x3.5 TeV collision; other events can be found here.

Read CERN Bulletin article in English and French and also CERN Courier article.

see LHCb video on YouTube, and CDS as well as interview with Tara Shears.

International

8.3.2010

On March 8th, during during International Women's Day, many many women have been present in the LHCb control roon, ...

Women's

8.3.2010

see the photos taken at the LHCb control room (click in pictures to it in higher resolution, click here to get other photos), the video interview with Monica Pepe-Altarelli here, ...

Day

8.3.2010

the LHCb women poster (click in image to get higher resolution), and the CERN and the Fermilab Web pages.

2009 News

Below you may find interesting computer reconstructed events observed with the LHCb detector during the LHC restart period in November and December 2009. Click on picture to see it in higher resolution version.

Protons have ended to circulate at LHC on December 16th.

1.2 TeV collisions at LHCb

14.12.09

On December 14th 1.2 TeV proton beams have collided at LHCb during LHC machine studies. Many LHCb subdetectors, except for sensitive silicon detectors, recorded the world's highest energy pp collisions. Other events can be found here.

proton interactions with gas

12.12.09

Position of proton interactions inside the vertex detector with residual gas are shown in blue or red, proton-proton interactions in green (more details).

K0 reconstruction

12.12.09

So called "strange particles" are produced in the proton-proton collisions and decay inside LHCb detector into two other particles reconstructed as red tracks (more details).

High multiplicity events

12.12.09

A high multiplicity event with three muon tracks (green) recorded on December 12th. Other events can be found here.

More proton-proton collisions

8.12.09

On December 8th many long tracks were reconstructed using the detectors along the whole length of the LHCb. Collision vertex is clearly observed (bottom left). The tracks are curved in the magnetic field allowing measurement of the track momentum (top left). Other events can be found here.

More proton-proton collisions

6.12.09

On December 6th protons have again collided at LHCb. Few modules of sensitive LHCb Vertex Locator VELO recorded tracks clearly indicating proton-proton collision vertex location (see picture left bottom). Other reconstructed events can be found here.

RICH rings

6.12.09

LHCb RICH detectors are used to identify particles. The circles show possible position of measured points for different kinds of particles traversing the detector. The measured points clearly choose one possibility for every circle and in this way allow to identify particles.

First proton-proton collisions

23.11.09

A proton-proton collision candidate event. On November 23 protons from two beams circulated at LHC and have collided at LHCb.

LHC news video youTube

First proton-proton collisions

23.11.09

LHCb control room at this historical moment.

LHCb November 23 pp collision video youTube, CDS

First proton-proton collisions

23.11.09

Tracks originate from the expected region inside LHCb Vertex Locator detector VELO.

First proton-proton collisions

23.11.09

Another proton-proton collision candidate event.

Annimation

23.11.09

Animation (click on picture): pp collisions and proton beam gas collisions recorded on Nov. 23, 2009. Other reconstructed events can be found here.

First reconstructed pi0's

23.11.09

pi0 is the short lived particle decaying into two photons which were measured in the electromagnetic calorimeter ECAL. The plot above shows the nearly perfectly reconstructed pi0 mass.

First proton interactions

22.11.09

... reconstructed Vertex Locator VELO track. Other reconstructed events can be found here.

First proton interactions

22.11.09

... event reconstructed later (offline reconstruction). Particularly interesting is the blue track recorded in the Vertex Locator VELO and in the tracking chambers after the magnet.

First proton interactions

21.11.09

... not yet proton-proton interaction but interactions of the protons with residual gas inside LHC ring. Tracks reconstructed during data taking.

A splash from the LHC beam

21.11.09

LHC has restarted on November 21st. Two LHC beams have made a full turn of the LHC. Afterwards, after synchronization with the LHC accelerating system (RF capture in technical language), the beams made few hundred turns. During LHC operation LHCb has recoded splash events. The movie (click on picture) shows what LHCb detector has recorded every 25ns (1/(40 000 000) s) for a particular splash event; see individual events here.

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