Characterisation and compensation of magnetic distortions for the pixel Hybrid Photon Detectors of the LHCb RICH

The LHCb experiment at LHC, optimised for the study of CP violation, has two RICH detectors to provide particle identification in the momentum range 2–100 GeV=c. The stringent requirements on the photon detectors are met by the custom-made pixel Hybrid Photon Detector. The photon detectors need to operate in the fringe field of the LHCb dipole magnet which will produce distortions of the image detected on the pixel chip which is encapsulated inside the HPD. This paper reports on the experimental characterisation of the image distortions caused by an external magnetic flux density. These measurements allow for the development of a parameterisation of the effects and a compensation algorithm, which are also presented.