Search for Proton Decay via $p \to e^+��^0$ and $p \to ��^+��^0$ in 0.31 megaton$\cdot$years exposure of the Super-Kamiokande Water Cherenkov Detector

We have searched for proton decay via $p \rightarrow e^{+} ��^{0}$ and $p \rightarrow ��^{+} ��^{0}$ using Super-Kamiokande data from April 1996 to March 2015, 0.306 megaton$\cdot$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 $p \rightarrow e^{+} ��^{0}$ search. Two candidates that passed all of the selection criteria for $p \rightarrow ��^{+} ��^{0}$ 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 $��/B(p \rightarrow e^{+} ��^{0}) > 1.6 \times 10^{34}$ years and $��/B(p \rightarrow ��^{+} ��^{0}) > 7.7 \times 10^{33}$ years at 90% confidence level.

11 pages, 5 figures