LHCb

• ATLAS
• CMS
• LHCb
• ALICE

• LHCf
• TOTEM

• MoEDAL

• Linear accelerators for protons ( p) and lead nuclei (Pb)
• Proton Synchrotron Booster (PSB )
• Proton Synchrotron (PS )
• Super Proton Synchrotron (SPS )

The LHCb experiment ( for the Large Hadron Collider beauty ) is one of six experiments at the Large Hadron Collider at CERN. LHCb is also specialized in the study of decays of hadrons containing a bottom or charm quark. The resulting precision measurements of CP violation and rare decays to allow sensitive tests of the Standard Model. Spokesman of the experiment is currently (2013 ) Pierluigi Campana.

Construction of the LHCb detector

The b- quarks are dominantly produced by the strong interaction, so that they are produced in pairs of quark and antiquark. Numerous measurements to neutral B mesons require knowledge as to whether the time of production was present a B or anti -b- curd. This often the decay products of both B hadrons have to be investigated in the event. Upon generation of a B- curd in the direction of the beam axis, the probability is maximum, that fly the other B- curd in this direction. This also explains the geometry of the LHCb detector, which is constructed as a forward spectrometer. For cost reasons, only one of the two possible directions is instrumented.

Like all major LHC detectors has also the LHCb detector via a beam monitor system ( Beam Conditions Monitor BCM). The BCM monitors the use of diamond sensors that are mounted close to the beam axis, the beam quality. The sensors measure the ionization generate charged particles in the passage. The signal exceeds certain thresholds, a beam abort is initiated automatically to protect the detector from damage caused by runaway rays.

VELO vertex detector

B mesons have a very short lifetime and disintegrate after only a few millimeters route. With the VELO detector an accurate position determination of the decay location is possible.

The vertex detector consists of 42 semi-circular semiconductor tracking detectors, which are arranged along the beam at the collision point. The semiconductor detectors with a thickness of 0.3 mm with a resolution of 10 microns and the next parts are located at a distance of only 7 mm to the beam. By this arrangement the collision points can be determined with a resolution of less than 50 microns.

During the testing phase and the filling of the LHC may cause instabilities of the jet. To protect the radiating near detectors in front of the high-energy beam, the detectors are mounted on the carriage and be driven only after the stabilization of the beam in the beam close, otherwise they are in the rest position 35 mm away from the beam. The whole VELO detector system is located in a vacuum chamber. The detectors are cooled to about -25 ° C with a CO2 refrigeration system.

RICH detectors

Directly behind the VELO detector is the RICH -1 detector. Is a ring Imaging Cerenkov detector in the Cerenkov radiation of the charged particles during the passage through an optically dense medium, the velocity of the particles can be determined.

In RICH -1, two optical media with different refractive indices are used, first a slice of airgel, followed by one with Perfluorobutane gas ( C4F10 ) filled space, bringing a wide pulse spectrum from 1 to 50 GeV / c can be measured. The Cherenkov radiation is passed out over two mirror systems from the beam path and is received by a system of 196 photodetectors.

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

Tracking system

The tracking system consists of the silicon detectors of the tracker Turicensis before the magnet and the wire chambers of the Outer Tracker or the silicon detectors of the Inner Tracker after the magnet. So that the flight path of the particles can be determined - it is possible to assign the trail behind the magnet track in front of the magnets, and receives information about the angle of deflection is a measure of the particle.

Calorimeter

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

Muon system

The last part of the detector are the muon chambers: These are specially designed for the detection of muons that arise in a number of important decays in the detector.

Trigger

The LHCb detector has a two-stage trigger system. In a first step hits are mainly used in the muon system and energy depositions in the calorimeter for decision making. With a rate of 1 MHz, the detector is read, and each event is processed on a computer farm on. The number of events is then reduced to about 5000 / s, which are then stored and available for further analysis.

Data acquisition

In the year 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 was at a center of mass energy of 8 TeV 2.082 fb- 1 already. The instantaneous luminosity is since 2011 if possible kept constant at 4 * 1032cm -2s -1 by suitably displacing the beam, which is about twice as large as originally planned. 2012 events at a rate of up to 5000 / s instead of the originally intended rate of 2000 / s have been stored.

Results

By September 2013 150 publications on results of the LHCb collaboration have already been published in refereed journals, covering a wide range of analysis issues.

Special attention learned the end of 2012 the search for the rare decay Bs → μ μ -. In November 2012, the collaboration was able to demonstrate for the first time this decay with a statistical significance of 3.5 σ. The measured branching ratio of ( 3.2 1.5-1.2 ) 10-9 agrees in excellent agreement with the prediction of the Standard Model. Through these measurements could be restricted numerous models of " new physics ". The CMS experiment could confirm this measurement since.

Unexpected was the discovery of CP violation in D mesons, which was significantly greater than the theoretical predictions. The interpretation is currently ( September 2013 ) still unclear, especially as a statistical fluctuation can not be excluded. In addition, the calculations seem to be still room for a slightly larger CP violation.

LHCb upgrade

The LHCb collaboration is planning an upgrade of the experiment in 2018 provided the second long shutdown of the LHC. The aim of these measures is a data acquisition at a higher instantaneous luminosity of 2 * 1033/cm2/s with this optimized detector and trigger. A special feature of the proposed trigger is that now all collision events are processed by the computer farm, killing events with hadronic B decays can be detected much more frequently, for example.