Neutron-Antineutron Oscillation Search using a 0.37 Megaton$cdot$Year Exposure of Super-Kamiokande

As a baryon number violating process with $Delta B=2$, neutron-antineutron oscillation ($ntobar n$) provides a unique test of baryon number conservation. We have performed a search for $ntobar n$ oscillation with bound neutrons in Super-Kamiokande, with the full data set from its first four run periods, representing an exposure of 0.37~Mton-years. The search used a multivariate analysis trained on simulated $ntobar n$ 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 $bar n$ appearance lifetime in $^{16}$O nuclei of $3.6times{10}^{32}$ years and on the neutron-antineutron oscillation time of $tau_{ntobar n} > 4.7times10^{8}$~s at 90% C.L..