Search for proton decay via $p o e^+pi^0$ and $p o mu^+pi^0$ in 0.31 megaton·years exposure of the Super-Kamiokande water Cherenkov detector

We have searched for proton decay via p -> e(+) pi(0) and p -> mu(+) pi(0) using Super-Kamiokande data from April 1996 to March 2015, 0.306 megaton center dot 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(+) pi(0) search. Two candidates that passed all of the selection criteria for p -> mu(+) pi(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 tau/B(p -> mu(+) pi(0)) > 1.6 x 10(34) years and tau/B(p -> mu(+) pi(0) ) > 7.7 x 10(33) years at 90% confidence level.