Search for proton decay via p → e~+π~0 and p → μ~+π~0 in 0.31 megaton · years exposure of the Super-Kamiokande water Cherenkov detector

摘要

We have searched for proton decay via p → e~+π~0 and p → μ~+π~0 using Super-Kamiokande data from April 1996 to March 2015, 0.306 megaton · years exposure in total. The atmospheric neutrino background rate in Super-Kamiokande IV is reduced to almost half that of phase I-III by tagging neutrons associated with neutrino interactions. The reach of the proton lifetime is further enhanced by introducing new signal criteria that select the decay of a proton in a hydrogen atom. No candidates were seen in the p → e~+π~0 search. Two candidates that passed all of the selection criteria for p → μ~+π~0 have been observed, but these are consistent with the expected number of background events of 0.87. Lower limits on the proton lifetime are set at τ/B(p → e~+π~0) > 1.6 × 10~(34) years and τ/B(p → μ~+π~0) > 7.7 × 10~(33) years at 90% confidence level.