Search for Nucleon Decay via $n\to \overline{\nu }{\pi }^0$ and $p\to \overline{\nu }{\pi }^{+}$ in Super-Kamiokande

We present the results of searches for nucleon decay via $n\to \overline{\nu }{\pi }^0$ and $p\to \overline{\nu }{\pi }^{+}$ using data from a combined $172.8\mathrm{kt}·\mathrm{yr}$ exposure of Super-Kamiokande-I,-II, and-III. We set lower limits on the partial lifetime for each of these modes: ${\tau }_{n\to \overline{\nu }{\pi }^0}>1.1\times {10}^{33}$ years and ${\tau }_{p\to \overline{\nu }{\pi }^{+}}>3.9\times {10}^{32}$ years at a 90% confidence level.

Figure 1

Event selection efficiencies for atmospheric neutrino MC as a function of reconstructed momentum for $n\to \overline{\nu }{\pi }^0$ selection cuts (top) and $p\to \overline{\nu }{\pi }^{+}$ selection cuts (bottom). For the ${\pi }^{+}$ sample, all momenta are calculated assuming a muon hypothesis. The atmospheric neutrino events that make up the background for the $p\to \overline{\nu }{\pi }^{+}$ mode are treated separately in the spectrum fit according to their origin.

Figure 2

Cumulative fractions of nuclear effects for ${\pi }^0$ as a function of true ${\pi }^0$ momentum for atmospheric neutrino interactions. The fractions of events that undergo charge exchange, multiple particle production, scattering, and absorption are shown by various shades as labeled. Pions that exit the nucleus without experiencing any nuclear effect are indicated by the portion labeled “No interaction.”

Figure 3

Reconstructed momentum for $172.8\mathrm{kt}·\mathrm{yr}$ of $\mathrm{SK}\text{−}\mathrm{I}+\mathrm{II}+\mathrm{III}$ data (black dots), best fit result of atmospheric neutrino plus nucleon decay MC simulation (solid line), and the 90% C.L. allowed amount of nucleon decay (hatched histogram) for $n\to \overline{\nu }{\pi }^0$ (top) and $p\to \overline{\nu }{\pi }^{+}$ (bottom). The dashed line shows how a positive signal of nucleon decay would look, corresponding to five times the limit we set on the decay partial lifetimes. The $p\to \overline{\nu }{\pi }^{+}$ nucleon decay contribution in the bottom figure is reconstructed at lower momentum than the expected value ($458.8\mathrm{MeV}/c$) because a muon hypothesis is assumed in the reconstruction.