LHCb

Large Hadron Collider (LHC) Arrangement of the various accelerators and detectors of the LHC
Detectors
ATLAS CMS LHCb ALICE	LHCf TOTEM  Partly built up: MoEDAL FIBER
Pre-accelerator
Linear accelerator for protons (p) or lead nuclei (Pb) Proton Synchrotron Booster (PSB) Proton Synchrotron (PS) Super Proton Synchrotron (SPS)

The LHCb experiment (for Large Hadron Collider beauty ) is one of six experiments at the Large Hadron Collider at CERN . LHCb specializes, among other things, in the investigation of the decay of hadrons containing a bottom or charm quark . The resulting precision measurements of CP violation or infrequent decays allow sensitive tests of the standard model . Since July 1, 2020, the spokesman for the experiment has been Chris Parkes , successor to Giovanni Passaleva (2017–2020 in this role).

Section through the LHCb detector

Structure of the LHCb detector

B mesons are mainly generated by processes of strong interaction . A b-quark plus a b-antiquark are created together. Measurements of neutral B- mesons require the knowledge whether a b-quark or b-antiquark was present at the time of generation. For this purpose, the decay products of both B mesons in the event are examined. If a b-quark is generated in the direction of the beam axis, the probability is maximum that the partner particle will also fly in this direction. This explains the geometry of the LHCb detector, which is designed as a forward spectrometer. For cost reasons, only one of the two possible directions is instrumented.

Like all large LHC detectors, the LHCb detector also has a beam monitoring system ( Beam Conditions Monitor , BCM). The BCM monitors the beam quality using diamond sensors that are mounted near the beam axis. The sensors measure the ionization that charged particles generate when they pass through. If the signal exceeds certain thresholds, the beam in the LHC is automatically directed out of the accelerator and disposed of ( beam dump ) in order to protect the detector from damage caused by out-of-control beams.

Vertex detector VELO

B mesons have a very short lifetime and decay after a few millimeters of flight. With the VELO detector it is possible to determine the exact position of the decay location and the particle tracks in the detector can be assigned to their original location.

The vertex detector consists of 42 semicircular semiconductor track detectors arranged along the beam around the collision point. The semiconductor detectors with a thickness of 0.3 mm each have a resolution of 10 µm and the next parts are only 7 mm away from the beam. With this arrangement, the collision points can be determined with a resolution of less than 50 µm.

The jet may become unstable during the test phase and when the LHC is being filled. In order to protect the detectors close to the beam from the high-energy beam, the detectors are mounted on slides and are only moved into the vicinity of the beam after the beam has stabilized, otherwise they are in the rest position 35 mm from the beam. The entire VELO detector system is located in a vacuum chamber. The detectors are cooled to about −25 ° C using a CO ₂ cooling system.

RICH detectors

The RICH-1 detector is located directly behind the VELO detector. It is a ring-imaging Cherenkov detector , in which the Cherenkov radiation of charged particles can be used to determine the speed of the particles as they pass through an optically dense medium.

In the RICH-1, two optical media with different refractive indices are used, first an airgel disk , followed by a space filled with perfluorobutane gas (C ₄ F ₁₀ ), with which a wide range of pulses from 1 to 50 GeV / c can be measured . The Cherenkov radiation is guided out of the beam path by two mirror systems and recorded by a system of 196 photodetectors.

The RICH-2 detector is located further back and is used to measure particles with higher momentum (up to about 150 GeV / c). In this way, the speed of the particles and thus also their mass can be measured over a large energy range, which is important for particle identification.

Tracking system

The tracking system consists of the silicon detectors of the Tracker Turicensis in front of the magnet and the wire chambers ( straw detector ) of the Outer Tracker or the silicon detectors of the Inner Tracker behind the magnet. This allows the trajectory of the particles to be determined ( tracking ) - you can assign the tracks in front of the magnet to the tracks behind the magnet and get a measurement of the particle momentum based on the deflection angle. The VELO data is also used for tracking.

calorimeter

Most of the particles are stopped in the calorimeters and their energy and direction of flight are determined again. This is especially important for uncharged particles, as these cannot be observed in the other parts of the detector. First electrons , positrons and photons are stopped in the electromagnetic calorimeter ( ECAL ), and then all hadrons are detected in the subsequent hadronic calorimeter ( HCAL ) .

Muon system

The last part of the detector is formed by the muon chambers: These are specially designed for the detection of muons , which are formed in the detector during some important decays.

Trigger

The LHCb detector has a two-stage trigger system. In a first step, hits in the muon system and energy deposits in the calorimeter are mainly used to make decisions, as these can be evaluated quickly. The first trigger stage reduces the event rate from 20 MHz to 1 MHz. The entire detector data is then read out at this rate and the respective event is processed further on a computer farm. The number of events is then reduced to approx. 5000 / s, which are then saved and available for further analysis.

Data recording

In 2010/2011, data were recorded at a center of mass energy of 7 TeV with an integrated luminosity of 1.145 fb ⁻¹ . In 2012 the integrated luminosity with a center of mass energy of 8 TeV was already 2.082 fb ⁻¹ . Since 2011, the instantaneous luminosity has been kept constant at 4 · 10 ³² cm ⁻² s ⁻¹ by appropriately shifting the rays , and is thus about twice as large as originally planned. In 2012, the events were saved at a rate of up to 5000 / s instead of the initially planned rate of 2000 / s.

Results

By September 2014, 219 publications on the results of the LHCb collaboration had already appeared in refereed journals, which cover a wide range of analysis topics.

The search for the rare decay B _s → μ ⁺ μ ^{- received} special attention at the end of 2012 . In November 2012, the collaboration was able to demonstrate this decay for the first time with a statistical significance of 3.5σ. The measured branching ratio of (3.2 ^+1.5 _−1.2 ) 10 ⁻⁹ agrees very well with the prediction of the Standard Model . Numerous “New Physics” models could be restricted through these measurements. The CMS experiment has since confirmed this measurement.

The measurement of CP violation in the case of D mesons , which turned out to be significantly larger than the theoretical predictions, was unexpected . The interpretation was initially unclear, especially since statistical fluctuation cannot be ruled out. In addition, the calculations also seem to leave room for a slightly larger CP violation. More recent measurements with a larger data set are compatible with the theoretical predictions.

On July 13, 2015, LHCb researchers reported the discovery of two pentaquark - charmonium states (pentaquarks with participation of charm and anti- charm quarks ) during the decay of the lambda-b baryon . ${\displaystyle \Lambda _{b}^{0}}$

In 2016, they announced the discovery of several tetraquarks , a type of exotic hadron that has been sought after for a long time.

Furthermore, several publications by LHCb indicate that lepton universality - the property that leptons do not differ in their interactions but only in their masses - could be violated. Further studies with larger data sets are needed to either clearly demonstrate this or to show that the older measurements were not based on a violation of lepton universality.

collaboration

The LHCb collaboration includes around 1350 scientists from 70 institutes in 17 countries (as of August 2015). Germany is represented by the universities of Aachen , Dortmund , Heidelberg and Rostock as well as the Max Planck Institute for Nuclear Physics , Switzerland by the University of Zurich and the ETH Lausanne .

LHCb upgrade

The LHCb collaboration is planning to upgrade the experiment in the second long LHC shutdown scheduled for 2018. The aim of these measures is to collect data at a higher instantaneous luminosity of 2 · 10 ³³ / cm ² / s with an optimized detector and trigger. A special feature of the planned trigger is that all collision events are now processed by the computer farm. B. Events with hadronic B decays can be recognized much more frequently.

See also

• MoEDAL

Web links

Commons : LHCb  - collection of images, videos and audio files

• LHCb Public Webpage
• LHCb on weltmaschine.de - the official website of the German LHC researchers
• LHCb section from US / LHC website
• Structure of the LHCb detector (German)
• Pictures of the construction of the VELO detector

Individual evidence

1. ^ Ana Lopes: New spokesperson for the LHCb collaboration. CERN, June 30, 2020, accessed July 3, 2020 . 
2. ↑ Giovanni Passaleva . Accessed December 4, 2017 . 
3. ^ C. Ilgner et al .: The Beam Conditions Monitor of the LHCb-Experiment . arxiv : 1001.2487
4. ^ The Large Hadron Collider Beauty experiment / VELO. CERN / LHCb, May 15, 2008, accessed March 20, 2010 . 
5. ^ The Large Hadron Collider Beauty experiment / RICH detectors. CERN / LHCb, May 15, 2008, accessed March 21, 2010 . 
6. ^ LHCb Operations Plots. CERN / LHCb, accessed January 10, 2016 . 
7. ↑ LHCb papers. LHCb, accessed September 20, 2014 . 
8. ↑ R. Aaij et al. (LHCb collaboration): First evidence for the decay B _s → μ ⁺ μ ^- . In: Phys. Rev. Lett. 110 (2013) 021801, arxiv : 1211.2674 .
9. ↑ CP violation in charm decays. LHCb, accessed February 10, 2013 . 
10. ↑ Th. Feldmann, S. Nanda, A. Soni: Repercussions of Flavor Symmetry Breaking on CP Violation in D-Meson Decays . arxiv : 1202.3795 .
11. ↑ Search for direct CP violation in D0 → h − h + modes using semileptonic B decays. LHCb collaboration, March 12, 2013, accessed September 20, 2014 . 
12. ↑ A search for time-integrated CP violation in D0 → K − K + and D0 → π − π + decays. LHCb collaboration, March 1, 2013, accessed September 20, 2014 . 
13. ↑ Observation of particles composed of five quarks, pentaquark-charmonium states, seen in decays. ${\displaystyle \Lambda _{b}^{0}\to J/\psi pK^{-}}$July 14, 2015, accessed July 14, 2015 . 
14. ^ LHCb collaboration: Observation of J / ψp resonances consistent with pentaquark states in decays ${\displaystyle \Lambda _{b}^{0}\to J/\psi K^{-}p}$ In: Phys. Rev. Lett. 115, 072001 (2015), arxiv : 1507.03414
15. ↑ An interesting result presented at the LHCP conference. June 3, 2014, accessed December 4, 2017 . 
16. ↑ LHCb collaboration: Test of lepton universality using decays ${\displaystyle B^{+}\to K^{+}\ell ^{+}\ell ^{-}}$ In: Phys. Rev. Lett. 113, 151601 (2014), arxiv : 1406.6482 .
17. ↑ To intriguing anomaly. May 25, 2015, accessed December 4, 2017 . 
18. ↑ LHCb collaboration: Measurement of the ratio of branching fractions ${\displaystyle {\mathcal {B}}({\overline {B}}^{0}\to D^{*+}\tau ^{-}{\overline {\nu }}_{\tau })/{\mathcal {B}}({\overline {B}}^{0}\to D^{*+}\mu ^{-}{\overline {\nu }}_{\mu })}$ In: Phys. Rev. Lett. 115, 111803 (2015), arxiv : 1506.08614 .
19. ^ Lepton universality test probes physics beyond the Standard Model. April 18, 2017, accessed December 4, 2017 . 
20. ↑ LHCb collaboration: Test of lepton universality with decays ${\displaystyle B^{0}\rightarrow K^{*0}\ell ^{+}\ell ^{-}}$ In: JHEP 08 (2017) 055, arxiv : 1705.05802 .
21. ^ New test of lepton universality. June 6, 2017, accessed December 4, 2017 . 
22. ^ First test of lepton universality using charmed beauty meson decays. September 13, 2017, accessed December 4, 2017 . 
23. ↑ LHCb collaboration: Measurement of the ratio of branching fractions ${\displaystyle {\mathcal {B}}(B_{c}^{+}\,\to \,J/\psi \tau ^{+}\nu _{\tau })/{\mathcal {B}}(B_{c}^{+}\,\to \,J/\psi \mu ^{+}\nu _{\mu })}$ In: arxiv : 1711.05623 .
24. ↑ List of participants , accessed on August 2, 2015
25. ↑ LHCb Collaboration as of Jan. 6, 2016. (No longer available online.) LHCb Collaboration, January 6, 2016, archived from the original on January 10, 2016 ; accessed on January 10, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
26. ^ Framework TDR for the LHCb Upgrade: Technical Design Report. CERN / LHCb, accessed February 3, 2013 .