CERN Accélérateur de science

1. Good trigger efficiencies for b-hadron final states with only hadrons, as well as those containing leptons.
2. Capability of identifying kaons and pions in a momentum range of $\sim 1$ to above 100 GeV/$c$.
3. Excellent decay time and mass resolution.

• Beam Pipe
  A 1.8 m-long section of the beam pipe around the interaction point has a large diameter of approximately 120 cm. This accommodates the vertex detector system with its retraction mechanics, and has a thin forward window made of aluminium over the full detector acceptance. This part is followed by two conical sections; the first is 1.5 m long with 25 mrad opening angle, and the second is 16 m long with 10 mrad opening angle. The entire first and most of the second section are made of beryllium in order to reduce the production of the secondary particles.
• Magnet
  A dipole magnet with the normal conductive Al coil provides a high field integral of 4 Tm. The polarity of the field can be changed to reduce systematic errors in the CP-violation measurements that could result from a left-right asymmetry of the detector.
• Vertex Locator
  A total of 21 stations made from two layers of silicon detector are used as a vertex detector system (VELO). Additional two stations with only one Si layer are dedicated to the detecting bunch crossings with more than one pp interaction as a part of Level-0 trigger. The closest distance between the active silicon area and the beam is 8 mm. The silicon detectors are placed in Roman pots with 300 $\mu$m thick aluminium windows, which act as a shield against RF pickup from the circulating beam bunches. In order to avoid collapse of the windows, a secondary vacuum is maintained inside the Roman pots. During the injection and acceleration, the Roman pot system will be moved away from the beam to avoid interference with the machine operation and accidental irradiation of the detectors.
• Tracking
  The LHCb tracking system consists of four stations; one upstream of the magnet (TT) and three just behind the magnet (T1 to T3). No tracking device is positioned in the magnet and most of the tracks are reconstructed by combining the VELO and tracking system. The first station is made of silicon detectors. The stations behind the magnet are split into Inner Tracker (IT) and Outer Tracker (OT) systems due to the high particle density close to the beam pipe. The IT system is made of Si, and drift chambers based on straw technology are used for the OT system.
• Ring Imaging Cherenkov Detectors
  The RICH system of the LHCb detector consists of two detectors with three different radiators in order to cover the required momentum range, 1-100 GeV/$c$ . The first detector uses aerogel and $\rm C_4 F_ {10}$ gas as radiators. The second detector, used for high momentum particles, is placed after the magnet and has $\rm C F_4$ gas as radiator. The Cherenkov light is detected with planes of Hybrid Photon Detectors (HPD's) placed outside the spectrometer acceptance.
• Calorimeters
  The calorimeter system consists of a preshower detector followed by electromagnetic and hadronic calorimeters. It also serves as the initial part of the muon filter system. The cells of the Preshower detector are made up from 12 mm-thick lead plates sandwiched by square scintillators, 15 mm thick. For the electromagnetic part a Shashlik calorimeter is used since only modest energy resolution is required. The hadron calorimeter is based on a scintillating tile design similar to that developed for the ATLAS experiment.
• Muon System
  The Muon system consists of tracking stations and absorber layers. The first tracking station is in front of the calorimeter system, which acts as the first absorber. Behind the calorimeter system, there are four tracking stations with Fe absorber walls in between. An additional Fe absorber is placed after the last tracking station against the muon background from the accelerator tunnel. Multi Wire Proportional Chambers are used everywhere except in the region close to the beam pipe of the first station where Triple-GEM chambers are used.
• Trigger
  The LHCb trigger has two decision levels. Using custom made electronics, the first decision is made based on high transverse momentum hadrons or electrons found in the calorimeter system, or muons found in the muon system, at the bunch crossing rate of 40 MHz. All data from the detector are then read out at a rate of 1 MHz and sent to a CPU farm for further event reduction. For this purpose, all the detector information is available. With a rate of 2 kHz, events which include calibration data are stored for offline analysis.

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