Magnetic distortion calibration for LHCb RICH detectors

The LHCb experiment will search for new physics in CP violation and rare decays from heavy flavour production at the LHC. Particle identification is provided by two Ring Imaging Cherenkov (RICH) detectors, which incorporate a total of 484 Hybrid Photon Detectors (HPDs).The RICH detectors are located in the fringe field of the LHCb dipole magnet and fields as large as 2.5 mT have been measured in some regions occupied by the RICH 1 photodetectors. It is necessary to make corrections for the distortion of the ring-images measured by the HPDs. RICH 1 incorporates a magnetic calibration system comprising a series of collimated LEDs mounted on x–y stages which scan over the front-faces of the HPD arrays. A similar system based on commercial projectors is used on RICH 2. The calibration is performed in dedicated runs with dipole field on and off. The calibration parameters are then stored in the Conditions Database and applied to the LHC collision data reconstruction. This report will present the improvement in Cherenkov angular resolutions with the magnetic calibration.