The performance of the LHCb Pixel Hybrid Photon Detectors in a 25ns structured test-beam

Particle identification plays an important role in the challenging physics programme of the LHCb experiment. Pion-kaon separation in the wide momentum range 1-100 GeV/$c$ will be achieved by a pair of Ring Imaging Cherenkov detectors, using three radiators. Cherenkov photons in the wavelength range 200-600 nm will be detected by pixel Hybrid Photon Detectors (HPDs) developed by LHCb in collaboration with industry. The HPDs and associated electronics have been designed to operate at 40 MHz, the bunch-crossing frequency of the LHC. Final production photon detectors and the full readout chain, coupled for the LHCb data handling system, have been tested for the first time in a particle beam, operating with the nominal 25 ns bunch spacing, in September 2006. A total of 48 HPDs mounted on three RICH columns have been installed inside a customised RICH detector using N$_2$ GeV/$c$ pion beam at the CERN Super Proton Synchrotron. The results of the tests demonstrate that the HPD is a reliable photon detector that meets the LHCb stringent requirements on photon detection efficiency and Cherenkov angle resolution.