Experimental study of the atmospheric neutrino backgrounds forp→e+π0searches in water Cherenkov detectors

The atmospheric neutrino background for proton decay via p→e + π 0 in ring imaging water Cherenkov detectors is studied with an artificial accelerator neutrino beam for the first time. In total, 3.14 × 10 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 π 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 + π 0 background rate by the KT data for SK from the atmospheric neutrinos whose energies are below 3 GeV is 1.63 +0.42 -0.33 (stat) +0.45 -0.51 (syst)(megaton-year) -1 . This result is also relevant to possible future, megaton-scale water Cherenkov detectors.