Experimental study of the atmospheric neutrino backgrounds for p→e+π0 searches in water Cherenkov detectors

The atmospheric neutrino background for proton decay via p -> e(+)pi(0) in ring imaging water Cherenkov detectors is studied with an artificial accelerator neutrino beam for the first time. In total, 3.14x10(5) neutrino events corresponding to about 10 megaton-years of atmospheric neutrino interactions were collected by a 1000 ton water Cherenkov detector (KT). The KT charged-current single pi(0) production data are well reproduced by simulation programs of neutrino and secondary hadronic interactions used in the Super-Kamiokande (SK) proton decay search. The obtained p -> e(+)pi(0) background rate by the KT data for SK from the atmospheric neutrinos whose energies are below 3 GeV is 1.63(-0.33)(+0.42)(stat)(-0.51)(+0.45)(syst)(megaton-year)(-1). This result is also relevant to possible future, megaton-scale water Cherenkov detectors.