Study of 1 megaton water cherenkov detectors for the future proton decay search

The sensitivity of a possible future 1 Megaton water Cherenkov detector for proton decay searches was studied. For p→e+π0 decay mode, the detection efficiency and the number of atmospheric neutrino backgrounds were estimated by using a detailed Monte Carlo simulation program. Moreover, their dependence on the number density of the photomultiplier tube (PMT) was investigated. With the PMT density of the Super-Kamiokande detector (2 PMT/m2, 40% photocathode coverage), we will reach to 1.5x1035 years partial lifetime limit at 90% confidence level by 10 years livetime of the detector (104 kton-year exposure). With a 1/4 (1/9) PMT density, the sensitivity for p→e+π0 mode is decreased to 1x1035 years (7x1034 years)