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{{short description|Experiment at the Large Hadron Collider}}{{coord|46|14|28|N|06|05|49|E|type:landmark|display=title}} {{LHC}}The LHCb (Large Hadron Collider beauty) experiment is a particle physics detector experiment collecting data at the Large Hadron Collider at CERN.JOURNAL, Belyaev, I., Carboni, G., Harnew, N., Teubert, C. Matteuzzi F., 2021-01-13, The history of LHCB, The European Physical Journal H, 46, 1, 3, 10.1140/epjh/s13129-021-00002-z, 2101.05331, 2021EPJH...46....3B, 231603240, LHCb is a specialized b-physics experiment, designed primarily to measure the parameters of CP violation in the interactions of b-hadrons (heavy particles containing a bottom quark). Such studies can help to explain the matter-antimatter asymmetry of the Universe. The detector is also able to perform measurements of production cross sections, exotic hadron spectroscopy, charm physics and electroweak physics in the forward region. The LHCb collaborators, who built, operate and analyse data from the experiment, are composed of approximately 1650 people from 98 scientific institutes, representing 22 countries.WEB,weblink LHCb Organization, Vincenzo VagnoniWEB, LHCb collaboration, 2023-07-05, New management for the LHCb collaboration in 2023,weblink 2024-02-05, CERN, en, succeeded on July 1, 2023 as spokesperson for the collaboration from Chris Parkes (spokesperson 2020–2023).WEB, New spokesperson for the LHCb collaboration,weblink 2024-02-05, LHCb, CERN, en, The experiment is located at point 8 on the LHC tunnel close to Ferney-Voltaire, France just over the border from Geneva. The (small) MoEDAL experiment shares the same cavern.

Physics goals

The experiment has wide physics program covering many important aspects of heavy flavour (both beauty and charm), electroweak and quantum chromodynamics (QCD) physics. Six key measurements have been identified involving B mesons. These are described in a roadmap documentARXIV , that formed the core physics programme for the first high energy LHC running in 2010–2012. They include:

• Measuring the branching ratio of the rare Bs → μ+ μ− decay.
• Measuring the forward-backward asymmetry of the muon pair in the flavour-changing neutral current Bd → K μ+ μ− decay. Such a flavour changing neutral current cannot occur at tree-level in the Standard Model of Particle Physics, and only occurs through box and loop Feynman diagrams; properties of the decay can be strongly modified by new physics.
• Measuring the CP violating phase in the decay Bs → J/ψ φ, caused by interference between the decays with and without Bs oscillations. This phase is one of the CP observables with the smallest theoretical uncertainty in the Standard Model, and can be significantly modified by new physics.
• Measuring properties of radiative B decays, i.e. B meson decays with photons in the final states. Specifically, these are again flavour-changing neutral current decays.
• Tree-level determination of the unitarity triangle angle γ.
• Charmless charged two-body B decays.

The LHCb detector

The fact that the two b-hadrons are predominantly produced in the same forward cone is exploited in the layout of the LHCb detector. The LHCb detector is a single arm forward spectrometer with a polar angular coverage from 10 to 300 milliradians (mrad) in the horizontal and 250 mrad in the vertical plane. The asymmetry between the horizontal and vertical plane is determined by a large dipole magnet with the main field component in the vertical direction.(File:Lhcb-logo-0121.svg|thumb|220x220px|The LHCb collaboration's logo)

Subsystems

The Vertex Locator (VELO) is built around the proton interaction region.weblink {{Webarchive|url=https://web.archive.org/web/20160303221602weblink|date=2016-03-03}}, The LHCb VELO (from the VELO group)weblink, VELO Public Pages It is used to measure the particle trajectories close to the interaction point in order to precisely separate primary and secondary vertices.The detector operates at {{convert|7|mm}} from the LHC beam. This implies an enormous flux of particles; VELO has been designed to withstand integrated fluences of more than 1014 p/cm2 per year for a period of about three years. The detector operates in vacuum and is cooled to approximately {{convert|-25|C|F}} using a biphase CO2 system. The data of the VELO detector are amplified and read out by the Beetle ASIC.{{Gallery }}The RICH-1 detector (Ring imaging Cherenkov detector) is located directly after the vertex detector. It is used for particle identification of low-momentum tracks.The main tracking system is placed before and after the dipole magnet. It is used to reconstruct the trajectories of charged particles and to measure their momenta. The tracker consists of three subdetectors:

• The Tracker Turicensis, a silicon strip detector located before the LHCb dipole magnet
• The Outer Tracker. A straw-tube based detector located after the dipole magnet covering the outer part of the detector acceptance
• The Inner Tracker, silicon strip based detector located after the dipole magnet covering the inner part of the detector acceptance

Following the tracking system is RICH-2. It allows the identification of the particle type of high-momentum tracks.The electromagnetic and hadronic calorimeters provide measurements of the energy of electrons, photons, and hadrons. These measurements are used at trigger level to identify the particles with large transverse momentum (high-Pt particles).The muon system is used to identify and trigger on muons in the events.

LHCb upgrade (2019–2021)

At the end of 2018, the LHC was shut down for upgrades, with a restart currently planned for early 2022. For the LHCb detector, almost all subdetectors are to be modernised or replaced.WEB, Transforming LHCb: What's in store for the next two years?,weblink 2021-03-21, CERN, en, It will get a fully new tracking system composed of a modernised vertex locator, upstream tracker (UT) and scintillator fibre tracker (SciFi). The RICH detectors will also be updated, as well as the whole detector electronics. However, the most important change is the switch to the fully software trigger of the experiment, which means that every recorded collision will be analysed by sophisticated software programmes without an intermediate hardware filtering step (which was found to be a bottleneck in the past).WEB, Allen initiative – supported by CERN openlab – key to LHCb trigger upgrade,weblink 2021-03-21, CERN, en,

Results

During the 2011 proton-proton run, LHCb recorded an integrated luminosity of 1 fb−1 at a collision energy of 7 TeV. In 2012, about 2 fb−1 was collected at an energy of 8 TeV.WEB,weblink Luminosities Run1, 14 Dec 2017, , 2012 LHC Luminosity Plots During 2015–2018 (Run 2 of the LHC), about 6 fb−1 was collected at a center-of-mass energy of 13 TeV. In addition, small samples were collected in proton-lead, lead-lead, and xenon-xenon collisions. The LHCb design also allowed the study of collisions of particle beams with a gas (helium or neon) injected inside the VELO volume, making it similar to a fixed-target experiment; this setup is usually referred to as "SMOG".WEB, 2020-05-08, New SMOG on the horizon,weblink 2021-03-21, CERN Courier, en-GB, These datasets allow the collaboration to carry out the physics programme of precision Standard Model tests with many additional measurements. As of 2021, LHCb has published more than 500 scientific papers.WEB, LHCb - Large Hadron Collider beauty experiment,weblink 2021-03-21, lhcb-public.web.cern.ch, en,

Hadron spectroscopy

LHCb is designed to study beauty and charm hadrons. In addition to precision studies of the known particles such as mysterious X(3872), a number of new hadrons have been discovered by the experiment. As of 2021, all four LHC experiments have discovered about 60 new hadrons in total, vast majority of which by LHCb.WEB, 59 new hadrons and counting,weblink 2021-03-21, CERN, en, In 2015, analysis of the decay of bottom lambda baryons (Λ{{su|p=0|b=b}}) in the LHCb experiment revealed the apparent existence of pentaquarks,WEB, 14 July 2015, Observation of particles composed of five quarks, pentaquark-charmonium states, seen in Λ{{su, 0, b, →J/ψpK− decays|url=http://lhcb-public.web.cern.ch/lhcb-public/Welcome.html#Penta|access-date=2015-07-14|publisher=CERN/LHCb}}JOURNAL, R. Aaij et al. (LHCb collaboration), 2015, Observation of J/ψp resonances consistent with pentaquark states in Λ{{su, 0, b, →J/ψK−p decays|journal=Physical Review Letters|volume=115|issue=7|pages=072001|arxiv=1507.03414|bibcode=2015PhRvL.115g2001A|doi=10.1103/PhysRevLett.115.072001|pmid=26317714|s2cid=119204136}} in what was described as an "accidental" discovery.WEB, G. Amit, 14 July 2015, Pentaquark discovery at LHC shows long-sought new form of matter,weblink 2015-07-14, New Scientist, Other notable discoveries are those of the "doubly charmed" baryon Xi_{rm cc}^{++} in 2017, being a first known baryon with two heavy quarks; and of the fully-charmed tetraquark mathrm{T}_{rm cccc} in 2020, made of two charm quarks and two charm antiquarks. {| class="wikitable mw-collapsible" Hadrons discovered at LHCb.WEB, New particles discovered at the LHC,weblink 2021-03-21, www.nikhef.nl, WEB,weblink Observation of a strange pentaquark, a doubly charged tetraquark and its neutral partner, The term 'excited' for baryons and mesons means existence of a state of lower mass with the same quark content and isospin. !!Quark content{{efn-lr|1=Abbreviations are the first letter of the quark name (up='u', down='d', top='t', bottom='b', charmed='c', strange='s'). Antiquarks have overbars.}}!Particle name!Type!Year of discovery|1|rm bud|Lambda_{rm b}(5912)^0|Excited baryon|2012|2|rm bud|Lambda_{rm b}(5920)^0|Excited baryon|2012|3|rm cbar{u}|rm D_J(2580)^0|Excited meson|2013|4|rm cbar{u}|rm D_J(2740)^0|Excited meson|2013|5|rm cbar{d}|rm D_J^*(2760)^+|Excited meson|2013|6|rm cbar{u}|rm D_J(3000)^0|Excited meson|2013|7|rm cbar{u}|rm D_J^*(3000)^0|Excited meson|2013|8|rm cbar{d}|rm D_J^*(3000)^+|Excited meson|2013|9|rm cbar{s}|rm D_{s1}^*(2860)^+|Excited meson|2014|10|rm bsd|Xi^{'-}_{rm b}|Excited baryon|2014|11|rm bsd|Xi^{*-}_{rm b}|Excited baryon|2014|12|rm bar{b}u|rm B_J(5840)^+|Excited meson|2015|13|rm bar{b}d|rm B_J(5840)^0|Excited meson|2015|14|rm bar{b}u|rm B_J(5970)^+|Excited meson|2015|15|rm bar{b}d|rm B_J(5970)^+|Excited meson|2015

Previously unknown combination of quarks}}|rm cbar{c}uud|rm P_c(4380)^+|Pentaquark|2015

|17|rm cbar{c}sbar{s}|rm X(4274)|Tetraquark|2016|18|rm cbar{c}sbar{s}|rm X(4500)|Tetraquark|2016|19|rm cbar{c}sbar{s}|rm X(4700)|Tetraquark|2016|20|rm cbar{u}|rm D_3^*(2760)^0|Excited meson|2016|21|rm cud|Lambda_{rm c}(2860)^+|Excited baryon|2017|22|rm css|Omega_{rm c}(3000)^0|Excited baryon|2017|23|rm css|Omega_{rm c}(3050)^0|Excited baryon|2017|24|rm css|Omega_{rm c}(3066)^0|Excited baryon|2017|25|rm css|Omega_{rm c}(3090)^0|Excited baryon|2017|26|rm css|Omega_{rm c}(3119)^0|Excited baryon|2017

Previously unknown combination of quarks; first baryon with two charm quarks, and the only weakly-decaying particle discovered so far at the LHC.}}|rm ccu|Xi_{rm cc}^{++}|Baryon|2017

|28|rm bsd|Xi_{rm b}(6227)^-|Excited baryon|2018|29|rm buu|Sigma_{rm b}(6097)^+|Excited baryon|2018|30|rm bdd|Sigma_{rm b}(6097)^-|Excited baryon|2018|31|rm cbar{c}

	ACCESS-DATE=2021-03-21, pdglive.lbl.gov, |Excited meson|2019

|32|rm cbar{c}uud|rm P_c(4312)^+|Pentaquark|2019|33|rm cbar{c}uud|rm P_c(4440)^+|Pentaquark|2019|34|rm cbar{c}uud|rm P_c(4457)^+|Pentaquark|2019|35|rm bud|Lambda_{rm b}(6146)^0|Excited baryon|2019|36|rm bud|Lambda_{rm b}(6152)^0|Excited baryon|2019|37|rm bss|Omega_{rm b}(6340)^-|Excited baryon|2020|38|rm bss|Omega_{rm b}(6350)^-|Excited baryon|2020

Simultaneous with CMS; CMS had not enough data to claim the discovery.}}|rm bud|Lambda_{rm b}(6070)^0|Excited baryon|2020

|40|rm csd|Xi_{rm c}(2923)^0|Excited baryon|2020|41|rm csd|Xi_{rm c}(2939)^0|Excited baryon|2020

Previously unknown combination of quarks; first tetraquark made exclusively of charm quarks}}|rm ccbar{c}bar{c}|rm T_{cccc}|Tetraquark|2020

Previously unknown combination of quarks; first tetraquark with all quarks being different}}|rm bar{c}dbar{s}u|rm X_0(2900)|Tetraquark|2020

|44|rm bar{c}dbar{s}u|rm X_1(2900)|Tetraquark|2020|45|rm bsu|Xi_{rm b}(6227)^0|Excited baryon|2020|46|rm bar{b}s|rm B_s(6063)^0|Excited meson|2020|47|rm bar{b}s|rm B_s(6114)^0|Excited meson|2020|48|rm cbar{s}|rm D_{s0}(2590)^+|Excited meson|2020|49|rm cbar{c}sbar{s}|rm X(4630)|Tetraquark|2021|50|rm cbar{c}sbar{s}|rm X(4685)|Tetraquark|2021|51|rm cbar{c}ubar{s}|rm Z_{cs}(4000)^+|Tetraquark|2021|52|rm cbar{c}ubar{s}|rm Z_{cs}(4220)^+|Tetraquark|2021{{notelist-lr}}

CP violation and mixing

Studies of charge-parity (CP) violation in B-meson decays is the primary design goal of the LHCb experiment. As of 2021, LHCb measurements confirm with a remarkable precision the picture described by the CKM unitarity triangle. The angle gamma , ,(alpha_3) of the unitarity triangle is now known to about 4°, and is in agreement with indirect determinations.BOOK,weblink Updated LHCb combination of the CKM angle γ, 2020, The LHCb Collaboration, In 2019, LHCb announced discovery of CP violation in decays of charm mesons.WEB, 2019-05-07, LHCb observes CP violation in charm decays,weblink 2021-03-21, CERN Courier, en-GB, This is the first time CP violation is seen in decays of particles other than kaons or B mesons. The rate of the observed CP asymmetry is at the upper edge of existing theoretical predictions, which triggered some interest among particle theorists regarding possible impact of physics beyond the Standard Model.JOURNAL, Dery, Avital, Nir, Yosef, December 2019, Implications of the LHCb discovery of CP violation in charm decays,weblink Journal of High Energy Physics, en, 2019, 12, 104, 10.1007/JHEP12(2019)104, 1909.11242, 2019JHEP...12..104D, 202750063, 1029-8479, In 2020, LHCb announced discovery of time-dependent CP violation in decays of Bs mesons.WEB, LHCb sees new form of matter–antimatter asymmetry in strange beauty particles,weblink 2021-03-21, CERN, en, The oscillation frequency of Bs mesons to its antiparticle and vice versa was measured to a great precision in 2021.

Rare decays

Rare decays are the decay modes harshly suppressed in the Standard Model, which makes them sensitive to potential effects from yet unknown physics mechanisms.In 2014, LHCb and CMS experiments published a joint paper in Nature announcing the discovery of the very rare decay mathrm{B}^0_{rm s} to mu^+mu^-, rate of which was found close to the Standard Model predictions.JOURNAL, Khachatryan, V., Sirunyan, A.M., Tumasyan, A., Adam, W., Bergauer, T., Dragicevic, M., Erö, J., Friedl, M., Frühwirth, R., Ghete, V.M., Hartl, C., June 2015, Observation of the rare B s 0 → μ + μ − decay from the combined analysis of CMS and LHCb data, Nature, en, 522, 7554, 68–72, 10.1038/nature14474, 26047778, 4394036, 1476-4687, free, 2445/195036, free, This measurement has harshly limited the possible parameter space of supersymmetry theories, which have predicted a large enhancement in rate. Since then, LHCb has published several papers with more precise measurements in this decay mode.Anomalies were found in several rare decays of B mesons. The most famous example in the so-called mathrm{P}_5^' angular observable was found in the decay mathrm{B}^0 to mathrm{K}^{*0} mu^+mu^-, where the deviation between the data and theoretical prediction has persisted for years.WEB, New LHCb analysis still sees previous intriguing results,weblink 2021-03-21, CERN, en, The decay rates of several rare decays also differ from the theoretical predictions, though the latter have sizeable uncertainties.

Lepton flavour universality{{anchor|Lepton_flavour_universality_anchor}}

{{See also|Lepton#Universality}}In the Standard Model, couplings of charged leptons (electron, muon and tau lepton) to the gauge bosons are expected to be identical, with the only difference emerging from the lepton masses. This postulate is referred to as "lepton flavour universality". As a consequence, in decays of b hadrons, electrons and muons should be produced at similar rates, and the small difference due to the lepton masses is precisely calculable.LHCb has found deviations from this predictions by comparing the rate of the decay mathrm{B}^+ to mathrm{K}^+ mu^+ mu^- to that of mathrm{B}^+ to mathrm{K}^+ mathrm{e}^+ mathrm{e}^-,WEB, How universal is (lepton) universality?,weblink 2021-03-21, CERN, en, and in similar processes.WEB, LHCb explores the beauty of lepton universality,weblink 2021-03-21, CERN, en, WEB, 2021-10-19, LHCb tests lepton universality in new channels,weblink 2021-10-27, CERN Courier, en-GB, However, as the decays in question are very rare, a larger dataset needs to be analysed in order to make definitive conclusions.In March 2021, LHCb announced that the anomaly in lepton universality crossed the "3 sigma" statistical significance threshold, which translates to a p-value of 0.1%.WEB, Intriguing new result from the LHCb experiment at CERN,weblink 2021-03-23, CERN, en, The measured value of R_{rm K} = frac{mathcal{B}(mathrm{B}^+ to mathrm{K}^+ mu^+mu^-)}{mathcal{B}(mathrm{B}^+ to mathrm{K}^+ mathrm{e}^+mathrm{e}^-)}, where symbol mathcal{B} denotes probability of a given decay to happen, was found to be 0.846^{+0.044}_{-0.041} while the Standard Model predicts it to be very close to unity.JOURNAL, LHCb collaboration, Aaij, R., Beteta, C. Abellán, Ackernley, T., Adeva, B., Adinolfi, M., Afsharnia, H., Aidala, C. A., Aiola, S., Ajaltouni, Z., Akar, S., 22 March 2022, Test of lepton universality in beauty-quark decays,weblink Nature Physics, en, 18, 3, 277–282, 2103.11769, 10.1038/s41567-021-01478-8, 2022NatPh..18..277L, 232307581, 1745-2473, In December 2022 improved measurements discarded this anomaly.JOURNAL, LHCb collaboration, 2023, Test of Lepton Universality in b → s ℓ+ ℓ− decays, Physical Review Letters, 131, 5, 051803, 10.1103/PhysRevLett.131.051803, 37595222, 2212.09152, 254854814, JOURNAL, LHCb collaboration, 2023, Measurement of lepton universality parameters in B+ → K+ ℓ+ ℓ− and B0→K∗0ℓ+ℓ− decays, Physical Review D, 108, 3, 032002, 10.1103/PhysRevD.108.032002, 2212.09153, 254853936, WEB, Improved lepton universality measurements show agreement with the Standard Model,weblink 2023-01-08, en-US, In August 2023 joined searches in leptonic decays brightarrow sell^+ell^- by the LHCb and semileptonic decays brightarrow sellnu by Belle II (with ell=e,mu) set new limits for universality violations.JOURNAL, Belle II Collaboration, Aggarwal, L., Ahmed, H., Aihara, H., Akopov, N., Aloisio, A., Anh Ky, N., Asner, D. M., Atmacan, H., Aushev, T., Aushev, V., Bae, H., Bahinipati, S., Bambade, P., Banerjee, Sw., 2023-08-02, Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic $B$-Meson Decays at Belle II,weblink Physical Review Letters, 131, 5, 051804, 10.1103/PhysRevLett.131.051804, 37595249, 2301.08266, 2023PhRvL.131e1804A, 256080428, JOURNAL, Wright, Katherine, 2023-08-02, Standard Model Stays Strong for Leptons,weblink Physics, en, 16, 5, s91, 10.1103/PhysRevLett.131.051804, 37595249, 2301.08266, 2023PhRvL.131e1804A, 256080428,

Other measurements

LHCb has contributed to studies of quantum chromodynamics, electroweak physics, and provided cross-section measurements for astroparticle physics.BOOK, Fontana, Marianna, 2017-10-19, LHCb inputs to astroparticle physics,weblink Proceedings of the European Physical Society Conference on High Energy Physics, 314, en, Venice, Italy, Sissa Medialab, 832, 10.22323/1.314.0832, free,

See also

{{Commons category|LHCb}}

• B-factory
• Belle II experiment

References

{{Reflist}}

External links

• {{Commons category-inline}}
• LHCb Public Webpage
• LHCb section from US/LHC Website {{Webarchive|url=https://web.archive.org/web/20200814045044weblink |date=2020-08-14 }}
• JOURNAL

• LHCb experiment record on INSPIRE-HEP

{{CERN}}