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

As a baryon number violating process with ΔB ¼ 2, neutron-antineutron oscillation (n → n¯) provides a unique test of baryon number conservation. We have performed a search for n → n¯ oscillation with bound neutrons in Super-Kamiokande, with the full dataset from its first four run periods, representing an exposure of 0.37 Mton-years. The search used a multivariate analysis trained on simulated n → 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 n¯ appearance lifetime in 16O nuclei of 3.6 × 1032 years and on the neutron-antineutron oscillation time of τn→n¯ > 4.7 × 108 s at 90% C.L.