The AQUA-RICH atmospheric neutrino experiment

We describe a 125 m diameter spherical detector containing 1 Mt of water, capable of high rate observation of atmospheric neutrino events (30000/y). The ring imaging Cherenkov (RICH) technique is used to measure velocity, momentum and direction of particles produced by neutrinos interacting in water. The detector will be sited outdoors (under a 50 m water shield) in a natural (further excavated) pit, probably in Sicily. Spherical reflecting mirrors focus Cherenkov light produced by secondaries from interacting neutrinos. Photons are detected by 5310 hybrid photodiodes (HPDs) of 1 m diameter each with 396 pads of 45*45 mm/sup 2/ on the photocathode surface, demagnified to 9*9 mm/sup 2/ on the silicon sensor. For most tracks the ring width will be dominated by multiple scattering which should allow momentum to be determined. Hadrons of momentum p<or=5 GeV/c can be measured with sigma /sub p//p<or=7and muons of p<or=32 GeV/c with sigma /sub p/<or=1The ring center determines track direction with sigma /sub 0x/ sigma /sub 0y/ approximately=6 mrad. Track reconstruction in water will require time resolution sigma /sub t/<1 ns. Detection of oscillating muon signals (disappearance) is feasible with atmospheric neutrinos and precision measurement of oscillation parameters is feasible for 10/sup -2/<or= Delta m/sup 2/<or=10/sup -4 / eV/sup 2/. Tau or sterile neutrino appearance experiments are also possible with atmospheric neutrinos. Other physics topics addressable with this detector are proton decay, supernova detection and search for astrophysical neutrino sources. A first test module of 3 tons water and 120 PMs (32 mm phi ) will operate (5/99) with 1-3 GeV muons in a CERN-PS test beam to verify the momentum algorithm. A second (6 m)/sup 3/ test module with 216 tons of water and 25 HPDs (0.25 m phi ) is designed to observe full multi-track images and test pattern recognition and ring reconstruction algorithms. (7 refs).