The calibration and alignment of the LHCb RICH system

Hadron identification in the LHCb experiment is performed by two Ring Imaging Cherenkov (RICH) detectors, comprising three radiators and 484 Hybrid Photon Detectors (HPDs). A well calibrated and aligned RICH system is essential for providing the particle identification performance necessary for realising the physics goals. The total number of mirrors in the two RICH detectors is 116 and they must be aligned to an accuracy of 0.1 mrad in order not to degrade the performance of the system. The refractive indices n of the two gas radiators, namely C4F10 and CF4, are expected to change with atmospheric pressure, and the refractivity ( n - 1 ) will be monitored to an accuracy of 0.1% from physics data. The Cherenkov angle resolutions and photon yields will be calibrated using saturated Cherenkov rings and particles of known type. Both RICH detectors operate in the fringe field of the LHCb dipole magnet, which produces image distortions of the HPDs. Solutions to correct for these distortions are in the implementation phase. Finally, the overall performance of the RICH system for hadron identification will be evaluated using samples of pions and kaons identified from D * decays, which can be isolated with high purity independently of the RICH itself.