Neutron-antineutron oscillation search using a 0.37 megaton-years exposure of Super-Kamiokande

As a baryon number violating process with $\mathrm{\ensuremath{\Delta}}B=2$, neutron-antineutron oscillation ($n\ensuremath{\rightarrow}\overline{n}$) provides a unique test of baryon number conservation. We have performed a search for $n\ensuremath{\rightarrow}\overline{n}$ oscillation with bound neutrons in Super-Kamiokande, with the full dataset from its first four run periods, representing an exposure of $0.37\text{ }\text{ }\mathrm{Mton}\text{\ensuremath{-}}\mathrm{years}$. The search used a multivariate analysis trained on simulated $n\ensuremath{\rightarrow}\overline{n}$ events and atmospheric neutrino backgrounds and resulted in 11 candidate events with an expected background of 9.3 events. In the absence of statistically significant excess, we derived a lower limit on $\overline{n}$ appearance lifetime in $^{16}\mathrm{O}$ nuclei of $3.6\ifmmode\times\else\texttimes\fi{}{10}^{32}\text{ }\text{ }\mathrm{years}$ and on the neutron-antineutron oscillation time of ${\ensuremath{\tau}}_{n\ensuremath{\rightarrow}\overline{n}}>4.7\ifmmode\times\else\texttimes\fi{}{10}^{8}\text{ }\text{ }\mathrm{s}$ at 90% C.L.