Search for Proton Decay via $p\to e^{+}{\pi }^0$ and $p\to {\mu }^{+}{\pi }^0$ in a Large Water Cherenkov Detector

We have searched for proton decays via $p\to e^{+}{\pi }^0$ and $p\to {\mu }^{+}{\pi }^0$ using data from a $91.7\mathrm{kt}·\mathrm{yr}$ exposure of Super-Kamiokande-I and a $49.2\mathrm{kt}·\mathrm{yr}$ exposure of Super-Kamiokande-II. No candidate events were observed with expected backgrounds induced by atmospheric neutrinos of 0.3 events for each decay mode. From these results, we set lower limits on the partial lifetime of $8.2\times {10}^{33}$ and $6.6\times {10}^{33}$ years at 90% confidence level for $p\to e^{+}{\pi }^0$ and $p\to {\mu }^{+}{\pi }^0$ modes, respectively.

Figure 1

Total momentum versus total invariant mass distributions for events of proton decay MC (a1),(b1), 500 yr-equivalent atmospheric neutrino MC (a2),(b2) and data (a3),(b3) which satisfy the selection criteria of $p\to e^{+}{\pi }^0$ except for (E) in SK-I (left figures) and SK-II (right figures). The boxes in figures indicates the criterion (E). Points in signal boxes of the atmospheric neutrino MC (a2),(b2) are shown in a larger size.

Figure 2

Total momentum versus total invariant mass distributions for events in SK-I (left) and SK-II (right) data, which satisfy the selection criteria of $p\to {\mu }^{+}{\pi }^0$ except for (E). The box in figures show the criterion (E).