LHCb experiment
The LHCb experiment is a particle physics detector collecting data at the Large Hadron Collider at CERN. LHCb specializes in the measurements of the parameters of CP violation in the interactions of b- and c-hadrons. 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, 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. Vincenzo Vagnoni succeeded on July 1, 2023 as spokesperson for the collaboration from Chris Parkes. The experiment is located at point 8 on the LHC tunnel close to Ferney-Voltaire, France just over the border from Geneva. The MoEDAL experiment shares the same cavern.
Physics goals
The experiment has wide physics program covering many important aspects of heavy flavour, electroweak and quantum chromodynamics physics. Six key measurements have been identified involving B mesons. These are described in a roadmap document 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
Image:Lhcbview.jpg|700px|LHCb detector along the bending plane
Subsystems
The Vertex Locator is built around the proton interaction region. 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 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 using a biphase CO2 system. The data of the VELO detector are amplified and read out by the Beetle ASIC.
The RICH-1 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
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.
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. It will get a fully new tracking system composed of a modernised vertex locator, upstream tracker and scintillator fibre tracker. 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.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. During 2015–2018, 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 injected inside the VELO volume, making it similar to a fixed-target experiment; this setup is usually referred to as "SMOG". 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.Hadron spectroscopy
LHCb is designed to study beauty and charm hadrons. In addition to precision studies of the known particles such as mysterious X, 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. In 2015, analysis of the decay of bottom lambda baryons in the LHCb experiment revealed the apparent existence of pentaquarks, in what was described as an "accidental" discovery. Other notable discoveries are those of the "doubly charmed" baryon in 2017, being a first known baryon with two heavy quarks; and of the fully-charmed tetraquark in 2020, made of two charm quarks and two charm antiquarks.| Quark content | Particle name | Type | Year of discovery | |
| 1 | Excited baryon | 2012 | ||
| 2 | Excited baryon | 2012 | ||
| 3 | Excited meson | 2013 | ||
| 4 | Excited meson | 2013 | ||
| 5 | Excited meson | 2013 | ||
| 6 | Excited meson | 2013 | ||
| 7 | Excited meson | 2013 | ||
| 8 | Excited meson | 2013 | ||
| 9 | Excited meson | 2014 | ||
| 10 | Excited baryon | 2014 | ||
| 11 | Excited baryon | 2014 | ||
| 12 | Excited meson | 2015 | ||
| 13 | Excited meson | 2015 | ||
| 14 | Excited meson | 2015 | ||
| 15 | Excited meson | 2015 | ||
| 16 | Pentaquark | 2015 | ||
| 17 | Tetraquark | 2016 | ||
| 18 | Tetraquark | 2016 | ||
| 19 | Tetraquark | 2016 | ||
| 20 | Excited meson | 2016 | ||
| 21 | Excited baryon | 2017 | ||
| 22 | Excited baryon | 2017 | ||
| 23 | Excited baryon | 2017 | ||
| 24 | Excited baryon | 2017 | ||
| 25 | Excited baryon | 2017 | ||
| 26 | Excited baryon | 2017 | ||
| 27 | Baryon | 2017 | ||
| 28 | Excited baryon | 2018 | ||
| 29 | Excited baryon | 2018 | ||
| 30 | Excited baryon | 2018 | ||
| 31 | Excited meson | 2019 | ||
| 32 | Pentaquark | 2019 | ||
| 33 | Pentaquark | 2019 | ||
| 34 | Pentaquark | 2019 | ||
| 35 | Excited baryon | 2019 | ||
| 36 | Excited baryon | 2019 | ||
| 37 | Excited baryon | 2020 | ||
| 38 | Excited baryon | 2020 | ||
| 39 | Excited baryon | 2020 | ||
| 40 | Excited baryon | 2020 | ||
| 41 | Excited baryon | 2020 | ||
| 42 | Tetraquark | 2020 | ||
| 43 | Tetraquark | 2020 | ||
| 44 | Tetraquark | 2020 | ||
| 45 | Excited baryon | 2020 | ||
| 46 | Excited meson | 2020 | ||
| 47 | Excited meson | 2020 | ||
| 48 | Excited meson | 2020 | ||
| 49 | Tetraquark | 2021 | ||
| 50 | Tetraquark | 2021 | ||
| 51 | Tetraquark | 2021 | ||
| 52 | Tetraquark | 2021 |