View source for LHCb experiment

{{short description|Experiment at the Large Hadron Collider}}
{{coord|46|14|28|N|06|05|49|E|type:landmark|display=title}} <!--verified on google maps-->
{{LHC}}
The '''LHCb''' ('''Large Hadron Collider beauty''') experiment is a particle physics detector collecting data at the [[Large Hadron Collider]] at [[CERN]].<ref>{{Cite journal|last1=Belyaev|first1=I.|last2=Carboni|first2=G.|last3=Harnew|first3=N.|last4=Teubert|first4=C. Matteuzzi F.|date=2021-01-13|title=The history of LHCB|journal=The European Physical Journal H|volume=46|issue=1|page=3|doi=10.1140/epjh/s13129-021-00002-z|arxiv=2101.05331|bibcode=2021EPJH...46....3B|s2cid=231603240}}</ref> LHCb specializes in the measurements of the parameters of [[CP violation]] in the interactions of b- and c-[[hadron]]s (heavy particles containing a [[bottom quark|bottom]] and [[charm quark|charm]] quarks). Such studies can help to explain the [[baryon asymmetry|matter-antimatter asymmetry]] of the Universe. The detector is also able to perform measurements of production cross sections, [[exotic hadron]] spectroscopy, and [[electroweak interaction|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.<ref>{{Cite web | url=https://lhcb.web.cern.ch/lhcb/lhcb_page/collaboration/organization/default.html | title=LHCb Organization}}</ref> Vincenzo Vagnoni<ref>{{Cite web |author=LHCb collaboration |date=2023-07-05 |title=New management for the LHCb collaboration in 2023 |url=https://home.cern/news/news/experiments/new-management-lhcb-collaboration-2023 |access-date=2024-02-05 |publisher=CERN |language=en}}</ref> succeeded on July 1, 2023 as spokesperson for the collaboration from Chris Parkes (spokesperson 2020–2023).<ref>{{Cite web |title=New spokesperson for the LHCb collaboration |url=https://home.cern/news/news/experiments/new-spokesperson-lhcb-collaboration |access-date=2024-02-05 |publisher=LHCb, CERN |language=en}}</ref> The experiment is located at point 8 on the LHC tunnel close to [[Ferney-Voltaire]], [[France]] just over the border from [[Geneva, Switzerland|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 (quantum number)|beauty]] and charm), electroweak and [[quantum chromodynamics]] (QCD) physics. Six key measurements have been identified involving B mesons. These are described in a roadmap document<ref>
{{cite arXiv
 |author=B. Adeva et al (LHCb collaboration)
 |year=2009
 |title=Roadmap for selected key measurements of LHCb
 |eprint=0912.4179
 |class=hep-ex
}}</ref> 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 B<sub>s</sub> → μ<sup>+</sup> μ<sup>−</sup> decay.
* Measuring the forward-backward asymmetry of the muon pair in the [[flavour-changing neutral current]] B<sub>d</sub> → K<sup>*</sup> μ<sup>+</sup> μ<sup>−</sup> 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 violation|CP violating]] phase in the decay B<sub>s</sub> → J/ψ φ, caused by interference between the decays with and without [[B–Bbar oscillation|B<sub>s</sub> 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 [[Cabibbo–Kobayashi–Maskawa matrix|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 [[milliradian]]s (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]]
[[Image:Lhcbview.jpg|700px|LHCb detector along the bending plane]]

=== Subsystems ===
The Vertex Locator (VELO) is built around the proton interaction region.<ref>[http://lhcb-vd.web.cern.ch/lhcb-vd/default.htm] {{Webarchive|url=https://web.archive.org/web/20160303221602/http://lhcb-vd.web.cern.ch/lhcb-vd/default.htm|date=2016-03-03}}, The LHCb VELO (from the VELO group)</ref><ref>[http://lhcb-public.web.cern.ch/lhcb-public/en/Detector/VELO-en.html], VELO Public Pages</ref> 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 10<sup>14</sup>&nbsp;p/cm<sup>2</sup> 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 [[carbon dioxide|CO<sub>2</sub>]] system. The data of the VELO detector are amplified and read out by the [[Beetle (ASIC)|Beetle ASIC]].
{{Gallery
  |title=VELO
  |align=center
  |File:VELO.jpg|The vertex locator (VELO) during construction.
  |File:VELO-IMG 5809-gradient.jpg|One element of VELO
}}

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 [[event reconstruction|reconstruct]] the trajectories of [[electric charge|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 [[electromagnetism|electromagnetic]] and [[hadron]]ic [[calorimeter (particle physics)|calorimeters]] provide measurements of the [[energy]] of [[electrons]], [[photons]], and [[hadrons]]. These measurements are used at [[trigger (particle physics)|trigger level]] to identify the particles with large transverse momentum (high-Pt particles).

The muon system is used to identify and [[trigger (particle physics)|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.<ref>{{Cite web|title=Transforming LHCb: What's in store for the next two years?|url=https://home.cern/news/news/experiments/transforming-lhcb-whats-store-next-two-years|access-date=2021-03-21|website=CERN|language=en}}</ref> 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).<ref>{{Cite web|title=Allen initiative – supported by CERN openlab – key to LHCb trigger upgrade|url=https://home.cern/news/news/computing/allen-initiative-supported-cern-openlab-key-lhcb-trigger-upgrade|access-date=2021-03-21|website=CERN|language=en}}</ref>

== Results ==
During the 2011 proton-proton run, LHCb recorded an integrated luminosity of 1 fb<sup>−1</sup> at a collision energy of 7 TeV. In 2012, about 2 fb<sup>−1</sup> was collected at an energy of 8 TeV.<ref>{{cite web |url=https://lhcb.web.cern.ch/lhcb/speakersbureau/html/Schematics/Luminosities_Run1.gif|title=Luminosities Run1|access-date=14 Dec 2017}}, 2012 LHC Luminosity Plots</ref> During 2015–2018 (Run 2 of the LHC), about 6 fb<sup>−1</sup> 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".<ref>{{Cite web|date=2020-05-08|title=New SMOG on the horizon|url=https://cerncourier.com/a/new-smog-on-the-horizon/|access-date=2021-03-21|website=CERN Courier|language=en-GB}}</ref> 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.<ref>{{Cite web|title=LHCb - Large Hadron Collider beauty experiment|url=http://lhcb-public.web.cern.ch/|access-date=2021-03-21|website=lhcb-public.web.cern.ch|language=en}}</ref>

=== Hadron spectroscopy ===
LHCb is designed to study beauty and charm [[hadron]]s. 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.<ref>{{Cite web|title=59 new hadrons and counting|url=https://home.cern/news/news/physics/59-new-hadrons-and-counting|access-date=2021-03-21|website=CERN|language=en}}</ref> In 2015, analysis of the decay of [[bottom lambda baryon]]s (Λ{{su|p=0|b=b}}) in the LHCb experiment revealed the apparent existence of [[pentaquark]]s,<ref name="LHCb-public">
{{cite web|date=14 July 2015|title=Observation of particles composed of five quarks, pentaquark-charmonium states, seen in Λ{{su|p=0|b=b}}→J/ψpK<sup>−</sup> decays|url=http://lhcb-public.web.cern.ch/lhcb-public/Welcome.html#Penta|access-date=2015-07-14|publisher=[[CERN]]/LHCb}}</ref><ref name="pentaquarkPRL">
{{cite journal|author=R. Aaij et al. (LHCb collaboration)|year=2015|title=Observation of J/ψp resonances consistent with pentaquark states in Λ{{su|p=0|b=b}}→J/ψK<sup>−</sup>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}}</ref> in what was described as an "accidental" discovery.<ref name="NewScientist2015">
{{cite web|author=G. Amit|date=14 July 2015|title=Pentaquark discovery at LHC shows long-sought new form of matter|url=https://www.newscientist.com/article/dn27892-pentaquark-discovery-at-lhc-shows-long-sought-new-form-of-matter/|access-date=2015-07-14|work=[[New Scientist]]}}</ref> Other notable discoveries are those of the "doubly charmed" baryon <math>\Xi_{\rm cc}^{++}</math> in 2017, being a first known [[baryon]] with two heavy quarks; and of the fully-charmed tetraquark <math>\mathrm{T}_{\rm cccc}</math> in 2020, made of two charm quarks and two charm antiquarks.   
{| class="wikitable mw-collapsible"
|+ class="nowrap"| Hadrons discovered at LHCb.<ref>{{Cite web|title=New particles discovered at the LHC|url=https://www.nikhef.nl/~pkoppenb/particles.html|access-date=2021-03-21|website=www.nikhef.nl}}</ref><ref>{{cite web | url=https://lhcb-outreach.web.cern.ch/2022/07/05/observation-of-a-strange-pentaquark-a-doubly-charged-tetraquark-and-its-neutral-partner/ | title=Observation of a strange pentaquark, a doubly charged tetraquark and its neutral partner }}</ref> 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 quark|up]]='u', [[down quark|down]]='d', [[top quark|top]]='t', [[bottom quark|bottom]]='b', [[charmed quark|charmed]]='c', [[strange quark|strange]]='s'). [[Antiquark]]s have overbars.}}
!Particle name
!Type
!Year of discovery
|-
|1
|<math>\rm bud</math>
|<math>\Lambda_{\rm b}(5912)^0</math>
|Excited baryon
|2012
|-
|2
|<math>\rm bud</math>
|<math>\Lambda_{\rm b}(5920)^0</math>
|Excited baryon
|2012
|-
|3
|<math>\rm c\bar{u}</math>
|<math>\rm D_J(2580)^0</math>
|Excited meson
|2013
|-
|4
|<math>\rm c\bar{u}</math>
|<math>\rm D_J(2740)^0</math>
|Excited meson
|2013
|-
|5
|<math>\rm c\bar{d}</math>
|<math>\rm D_J^*(2760)^+</math>
|Excited meson
|2013
|-
|6
|<math>\rm c\bar{u}</math>
|<math>\rm D_J(3000)^0</math>
|Excited meson
|2013
|-
|7
|<math>\rm c\bar{u}</math>
|<math>\rm D_J^*(3000)^0</math>
|Excited meson
|2013
|-
|8
|<math>\rm c\bar{d}</math>
|<math>\rm D_J^*(3000)^+</math>
|Excited meson
|2013
|-
|9
|<math>\rm c\bar{s}</math>
|<math>\rm D_{s1}^*(2860)^+</math>
|Excited meson
|2014
|-
|10
|<math>\rm bsd</math>
|<math>\Xi^{'-}_{\rm b}</math>
|Excited baryon
|2014
|-
|11
|<math>\rm bsd</math>
|<math>\Xi^{*-}_{\rm b}</math>
|Excited baryon
|2014
|-
|12
|<math>\rm \bar{b}u</math>
|<math>\rm B_J(5840)^+</math>
|Excited meson
|2015
|-
|13
|<math>\rm \bar{b}d</math>
|<math>\rm B_J(5840)^0</math>
|Excited meson
|2015
|-
|14
|<math>\rm \bar{b}u</math>
|<math>\rm B_J(5970)^+</math>
|Excited meson
|2015
|-
|15
|<math>\rm \bar{b}d</math>
|<math>\rm B_J(5970)^+</math>
|Excited meson
|2015
|-
|16{{efn-lr|Previously unknown combination of quarks}}
|<math>\rm c\bar{c}uud</math>
|<math>\rm P_c(4380)^+</math>
|Pentaquark
|2015
|-
|17
|<math>\rm c\bar{c}s\bar{s}</math>
|<math>\rm X(4274)</math>
|Tetraquark
|2016
|-
|18
|<math>\rm c\bar{c}s\bar{s}</math>
|<math>\rm X(4500)</math>
|Tetraquark
|2016
|-
|19
|<math>\rm c\bar{c}s\bar{s}</math>
|<math>\rm X(4700)</math>
|Tetraquark
|2016
|-
|20
|<math>\rm c\bar{u}</math>
|<math>\rm D_3^*(2760)^0</math>
|Excited meson
|2016
|-
|21
|<math>\rm cud</math>
|<math>\Lambda_{\rm c}(2860)^+</math>
|Excited baryon
|2017
|-
|22
|<math>\rm css</math>
|<math>\Omega_{\rm c}(3000)^0</math>
|Excited baryon
|2017
|-
|23
|<math>\rm css</math>
|<math>\Omega_{\rm c}(3050)^0</math>
|Excited baryon
|2017
|-
|24
|<math>\rm css</math>
|<math>\Omega_{\rm c}(3066)^0</math>
|Excited baryon
|2017
|-
|25
|<math>\rm css</math>
|<math>\Omega_{\rm c}(3090)^0</math>
|Excited baryon
|2017
|-
|26
|<math>\rm css</math>
|<math>\Omega_{\rm c}(3119)^0</math>
|Excited baryon
|2017
|-
|27{{efn-lr|Previously unknown combination of quarks; first baryon with two charm quarks, 
and the only [[Weak interaction|weakly]]-decaying particle discovered so far at the LHC.}}
|<math>\rm ccu</math>
|<math>\Xi_{\rm cc}^{++}</math>
|Baryon
|2017
|-
|28
|<math>\rm bsd</math>
|<math>\Xi_{\rm b}(6227)^-</math>
|Excited baryon
|2018
|-
|29
|<math>\rm buu</math>
|<math>\Sigma_{\rm b}(6097)^+</math>
|Excited baryon
|2018
|-
|30
|<math>\rm bdd</math>
|<math>\Sigma_{\rm b}(6097)^-</math>
|Excited baryon
|2018
|-
|31
|<math>\rm c\bar{c}</math>
|<math>\psi_3 (3842)</math><ref>{{Cite web|title=pdgLive|url=https://pdglive.lbl.gov/Particle.action?init=0&node=M241&home=MXXX025|access-date=2021-03-21|website=pdglive.lbl.gov}}</ref>
|Excited meson
|2019
|-
|32
|<math>\rm c\bar{c}uud</math>
|<math>\rm P_c(4312)^+</math>
|Pentaquark
|2019
|-
|33
|<math>\rm c\bar{c}uud</math>
|<math>\rm P_c(4440)^+</math>
|Pentaquark
|2019
|-
|34
|<math>\rm c\bar{c}uud</math>
|<math>\rm P_c(4457)^+</math>
|Pentaquark
|2019
|-
|35
|<math>\rm bud</math>
|<math>\Lambda_{\rm b}(6146)^0</math>
|Excited baryon
|2019
|-
|36
|<math>\rm bud</math>
|<math>\Lambda_{\rm b}(6152)^0</math>
|Excited baryon
|2019
|-
|37
|<math>\rm bss</math>
|<math>\Omega_{\rm b}(6340)^-</math>
|Excited baryon
|2020
|-
|38
|<math>\rm bss</math>
|<math>\Omega_{\rm b}(6350)^-</math>
|Excited baryon
|2020
|-
|39{{efn-lr|Simultaneous with [[Compact Muon Solenoid|CMS]]; CMS had not enough data to claim the discovery.}}
|<math>\rm bud</math>
|<math>\Lambda_{\rm b}(6070)^0</math>
|Excited baryon
|2020
|-
|40
|<math>\rm csd</math>
|<math>\Xi_{\rm c}(2923)^0</math>
|Excited baryon
|2020
|-
|41
|<math>\rm csd</math>
|<math>\Xi_{\rm c}(2939)^0</math>
|Excited baryon
|2020
|-
|42{{efn-lr|Previously unknown combination of quarks; first tetraquark made exclusively of charm quarks}}
|<math>\rm cc\bar{c}\bar{c}</math>
|<math>\rm T_{cccc}</math>
|Tetraquark
|2020
|-
|43{{efn-lr|Previously unknown combination of quarks; first tetraquark with all quarks being different}}
|<math>\rm \bar{c}d\bar{s}u</math>
|<math>\rm X_0(2900)</math>
|Tetraquark
|2020
|-
|44
|<math>\rm \bar{c}d\bar{s}u</math>
|<math>\rm X_1(2900)</math>
|Tetraquark
|2020
|-
|45
|<math>\rm bsu</math>
|<math>\Xi_{\rm b}(6227)^0</math>
|Excited baryon
|2020
|-
|46
|<math>\rm \bar{b}s</math>
|<math>\rm B_s(6063)^0</math>
|Excited meson
|2020
|-
|47
|<math>\rm \bar{b}s</math>
|<math>\rm B_s(6114)^0</math>
|Excited meson
|2020
|-
|48
|<math>\rm c\bar{s}</math>
|<math>\rm D_{s0}(2590)^+</math>
|Excited meson
|2020
|-
|49
|<math>\rm c\bar{c}s\bar{s}</math>
|<math>\rm X(4630)</math>
|Tetraquark
|2021
|-
|50
|<math>\rm c\bar{c}s\bar{s}</math>
|<math>\rm X(4685)</math>
|Tetraquark
|2021
|-
|51
|<math>\rm c\bar{c}u\bar{s}</math>
|<math>\rm Z_{cs}(4000)^+</math>
|Tetraquark
|2021
|-
|52
|<math>\rm c\bar{c}u\bar{s}</math>
|<math>\rm Z_{cs}(4220)^+</math>
|Tetraquark
|2021
|}
{{notelist-lr}}

=== CP violation and mixing ===
Studies of [[CP violation|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 <math>\gamma  \, \,(\alpha_3)</math> of the unitarity triangle is now known to about 4°, and is in agreement with indirect determinations.<ref>{{Cite book|url=https://cds.cern.ch/record/2743058|title=Updated LHCb combination of the CKM angle &gamma;|date=2020|editor-last=The LHCb Collaboration}}</ref>

In 2019, LHCb announced discovery of CP violation in decays of charm mesons.<ref>{{Cite web|date=2019-05-07|title=LHCb observes CP violation in charm decays|url=https://cerncourier.com/a/lhcb-observes-cp-violation-in-charm-decays/|access-date=2021-03-21|website=CERN Courier|language=en-GB}}</ref> 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.<ref>{{Cite journal|last1=Dery|first1=Avital|last2=Nir|first2=Yosef|date=December 2019|title=Implications of the LHCb discovery of CP violation in charm decays|url=http://link.springer.com/10.1007/JHEP12(2019)104|journal=Journal of High Energy Physics|language=en|volume=2019|issue=12|pages=104|doi=10.1007/JHEP12(2019)104|arxiv=1909.11242|bibcode=2019JHEP...12..104D|s2cid=202750063|issn=1029-8479}}</ref>

In 2020, LHCb announced discovery of time-dependent CP violation in decays of B<sub>s</sub> mesons.<ref>{{Cite web|title=LHCb sees new form of matter–antimatter asymmetry in strange beauty particles|url=https://home.cern/news/news/physics/lhcb-sees-new-form-matter-antimatter-asymmetry-strange-beauty-particles|access-date=2021-03-21|website=CERN|language=en}}</ref>  The oscillation frequency of B<sub>s</sub> 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 [[Compact Muon Solenoid|CMS]] experiments published a joint paper in [[Nature (journal)|Nature]] announcing the discovery of the very rare decay <math>\mathrm{B}^0_{\rm s} \to \mu^+\mu^-</math>, rate of which was found close to the Standard Model predictions.<ref>{{Cite journal|last1=Khachatryan|first1=V.|last2=Sirunyan|first2=A.M.|last3=Tumasyan|first3=A.|last4=Adam|first4=W.|last5=Bergauer|first5=T.|last6=Dragicevic|first6=M.|last7=Erö|first7=J.|last8=Friedl|first8=M.|last9=Frühwirth|first9=R.|last10=Ghete|first10=V.M.|last11=Hartl|first11=C.|date=June 2015|title=Observation of the rare B s 0 → μ + μ − decay from the combined analysis of CMS and LHCb data|journal=Nature|language=en|volume=522|issue=7554|pages=68–72|doi=10.1038/nature14474|pmid=26047778|s2cid=4394036|issn=1476-4687|doi-access=free|hdl=2445/195036|hdl-access=free}}</ref> 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 <math>\mathrm{P}_5^'</math> angular observable was found in the decay <math>\mathrm{B}^0 \to \mathrm{K}^{*0} \mu^+\mu^-</math>, where the deviation between the data and theoretical prediction has persisted for years.<ref>{{Cite web|title=New LHCb analysis still sees previous intriguing results|url=https://home.cern/news/news/physics/new-lhcb-analysis-still-sees-previous-intriguing-results|access-date=2021-03-21|website=CERN|language=en}}</ref> 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 [[lepton]]s (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 <math>\mathrm{B}^+ \to \mathrm{K}^+ \mu^+ \mu^-</math> to that of <math>\mathrm{B}^+ \to \mathrm{K}^+  \mathrm{e}^+ \mathrm{e}^-</math>,<ref>{{Cite web|title=How universal is (lepton) universality?|url=https://home.cern/news/news/accelerators/how-universal-lepton-universality|access-date=2021-03-21|website=CERN|language=en}}</ref> and in similar processes.<ref>{{Cite web|title=LHCb explores the beauty of lepton universality|url=https://home.cern/news/news/physics/lhcb-explores-beauty-lepton-universality|access-date=2021-03-21|website=CERN|language=en}}</ref><ref>{{Cite web|date=2021-10-19|title=LHCb tests lepton universality in new channels|url=https://cerncourier.com/a/lhcb-tests-lepton-universality-in-new-channels/|access-date=2021-10-27|website=CERN Courier|language=en-GB}}</ref> 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 [[Standard deviation|sigma]]" [[statistical significance]] threshold, which translates to a [[p-value]] of 0.1%.<ref>{{Cite web|title=Intriguing new result from the LHCb experiment at CERN|url=https://home.cern/news/news/physics/intriguing-new-result-lhcb-experiment-cern|access-date=2021-03-23|website=CERN|language=en}}</ref> The measured value of <math>R_{\rm K} = \frac{\mathcal{B}(\mathrm{B}^+ \to \mathrm{K}^+ \mu^+\mu^-)}{\mathcal{B}(\mathrm{B}^+ \to \mathrm{K}^+ \mathrm{e}^+\mathrm{e}^-)}</math>, where symbol <math>\mathcal{B}</math> denotes probability of a given decay to happen, was found to be <math>0.846^{+0.044}_{-0.041}</math> while the Standard Model predicts it to be very close to unity.<ref name="LHCb 2022">{{Cite journal |last1=LHCb collaboration |last2=Aaij |first2=R. |last3=Beteta |first3=C. Abellán |last4=Ackernley |first4=T. |last5=Adeva |first5=B. |last6=Adinolfi |first6=M. |last7=Afsharnia |first7=H. |last8=Aidala |first8=C. A. |last9=Aiola |first9=S. |last10=Ajaltouni |first10=Z. |last11=Akar |first11=S. |date=22 March 2022 |title=Test of lepton universality in beauty-quark decays |url=https://www.nature.com/articles/s41567-021-01478-8 |journal=Nature Physics |language=en |volume=18 |issue=3 |pages=277–282 |arxiv=2103.11769 |doi=10.1038/s41567-021-01478-8 |bibcode=2022NatPh..18..277L |s2cid=232307581 |issn=1745-2473}}</ref> In December 2022 improved measurements discarded this anomaly.<ref name="LHCb 2023B">{{Cite journal |last=LHCb collaboration |date=2023 |title=Test of Lepton Universality in ''b'' → ''s'' ℓ<sup>+</sup> ℓ<sup>−</sup> decays |journal=Physical Review Letters |volume=131 |issue=5 |page=051803 |doi=10.1103/PhysRevLett.131.051803 |pmid=37595222 |arxiv=2212.09152 |s2cid=254854814 }}</ref><ref name="LHCB 2023A">{{Cite journal |last=LHCb collaboration |date=2023 |title=Measurement of lepton universality parameters in ''B''<sup>+</sup> → ''K''<sup>+</sup> ℓ<sup>+</sup> ℓ<sup>−</sup> and ''B''<sup>0</sup>→''K''<sup>∗0</sup>ℓ<sup>+</sup>ℓ<sup>−</sup> decays |journal=Physical Review D |volume=108 |issue=3 |page=032002 |doi=10.1103/PhysRevD.108.032002 |arxiv=2212.09153 |s2cid=254853936 }}</ref><ref>{{Cite web |title=Improved lepton universality measurements show agreement with the Standard Model |url=https://lhcb-outreach.web.cern.ch/2022/12/20/improved-lepton-universality-measurements-show-agreement-with-the-standard-model/ |access-date=2023-01-08 |language=en-US}}</ref>

In August 2023 joined searches in leptonic decays <math>b\rightarrow s\ell^+\ell^-</math> by the LHCb and semileptonic decays <math>b\rightarrow s\ell\nu</math> by Belle II (with <math>\ell=e,\mu</math>) set new limits for universality violations.
<ref  name="LHCb 2023B" /><ref name="LHCB 2023A" /><ref>{{Cite journal |last1=Belle II Collaboration |last2=Aggarwal |first2=L. |last3=Ahmed |first3=H. |last4=Aihara |first4=H. |last5=Akopov |first5=N. |last6=Aloisio |first6=A. |last7=Anh Ky |first7=N. |last8=Asner |first8=D. M. |last9=Atmacan |first9=H. |last10=Aushev |first10=T. |last11=Aushev |first11=V. |last12=Bae |first12=H. |last13=Bahinipati |first13=S. |last14=Bambade |first14=P. |last15=Banerjee |first15=Sw. |date=2023-08-02 |title=Test of Light-Lepton Universality in the Rates of Inclusive Semileptonic $B$-Meson Decays at Belle II |url=https://link.aps.org/doi/10.1103/PhysRevLett.131.051804 |journal=Physical Review Letters |volume=131 |issue=5 |pages=051804 |doi=10.1103/PhysRevLett.131.051804|pmid=37595249 |arxiv=2301.08266 |bibcode=2023PhRvL.131e1804A |s2cid=256080428 }}</ref><ref>{{Cite journal |last=Wright |first=Katherine |date=2023-08-02 |title=Standard Model Stays Strong for Leptons |url=https://physics.aps.org/articles/v16/s91 |journal=Physics |language=en |volume=16 |issue=5 |pages=s91 |doi=10.1103/PhysRevLett.131.051804|pmid=37595249 |arxiv=2301.08266 |bibcode=2023PhRvL.131e1804A |s2cid=256080428 }}</ref>

=== Other measurements ===
LHCb has contributed to studies of quantum chromodynamics, electroweak physics, and provided cross-section measurements for astroparticle physics.<ref>{{Cite book|last=Fontana|first=Marianna|date=2017-10-19|chapter=LHCb inputs to astroparticle physics|chapter-url=https://pos.sissa.it/314/832|title=Proceedings of the European Physical Society Conference on High Energy Physics|volume=314 |language=en|location=Venice, Italy|publisher=Sissa Medialab|pages=832|doi=10.22323/1.314.0832 |doi-access=free }}</ref>

==See also==
{{Commons category|LHCb}}
*[[B-factory]]
*[[Belle II experiment]]

== References ==
{{Reflist}}

==External links==
*{{Commons category-inline}}
*[https://lhcb-outreach.web.cern.ch/ LHCb Public Webpage]
*[http://united-states.cern/accelerators-and-detectors/lhcb-experiment LHCb section from US/LHC Website] {{Webarchive|url=https://web.archive.org/web/20200814045044/http://united-states.cern/accelerators-and-detectors/lhcb-experiment |date=2020-08-14 }}
* {{cite journal
 |author=A. Augusto Alves Jr. ''et al.'' (LHCb Collaboration)
 |year=2008
 |title=The LHCb Detector at the LHC
 |url=http://www.iop.org/EJ/journal/-page=extra.lhc/jinst
 |journal=[[Journal of Instrumentation]]
 |volume=3 |pages=S08005
 |doi=10.1088/1748-0221/3/08/S08005
 |bibcode = 2008JInst...3S8005L
 |issue=8 |hdl=10251/54510
 |s2cid=250673998
 |hdl-access=free
 }} (Full design documentation)
* [https://inspirehep.net/experiments/1110643 LHCb experiment] record on [[INSPIRE-HEP]]

{{CERN}}

[[Category:CERN experiments]]
[[Category:Particle experiments]]
[[Category:Large Hadron Collider]]
[[Category:B physics]]