Performance of the LHCb RICH photodetectors in a charged particle beam

The Ring Imaging Cherenkov detectors of LHCb will use pixel Hybrid Photon Detectors to measure the spatial position of Cherenkov photons. The first six pre-production photon detectors have been tested in a beam, together with prototypes of the on-detector electronics. The tests were performed at CERN using pions together with an N2 gas radiator as a source of Cherenkov light. With 1.1 m of radiator, around 10 photoelectrons were detected per track. The single-photon Cherenkov angle resolution was measured to be , which is dominated by the pixelisation of the photon detector in the test-beam set-up. Both numbers agree with expectations.