A long base line RICH with a 27 kton water target and radiator for detection of neutrino oscillations

A 27 kton water volume is considered as a target for a long base line neutrino beam from CERN to Gran Sasso. Charged secondaries from the neutrino interactions produce Cherenkov photons in water which are imaged as rings by a sphericalmirror. The photon detector elements are 14400 photomultipliers (PMs) of 127 mm diameter with single photon sensitivity. A coincidence signal of approximate to 300 PMs in time with the SPS beam burst starts readout of the PMs in bins of 1 ns over a period of 100 ns. This defines the effective detector granularity to be 1.44 Mpixels, quite sufficient for the maximum expected event size of approximate to 12 x 104 photon hit points. Momentum, direction and velocity of hadrons and muons are determined from the width, center and radius of the rings, respectively. Momentum is measured if multiple scattering dominates the ring width, as is the case for most of the particles of interest. Thresholds in water for muons, pions, kaons and protons are 0.12, 0.16, 0.55 and 1.05 GeV/c, respectively. Momentum resolutions of 1-10%, mass resolutions of 5-50 MeV and direction resolutions of <1 mrad, are achievable. Electrons and gammas can be measured with energy resolution sigma(E)/E approximate to 7%/ root E(GeV) and with direction resolution approximate to 1 mrad. The detector can be sited above ground because it is directional and the SPS beam is pulsed thus it has excellent rejection of cosmic ray background.