As a baryon number violating process with $\mathrm{\Delta }B=2$, neutron-antineutron oscillation ($n\to \overline{n}$) provides a unique test of baryon number conservation. We have performed a search for $n\to \overline{n}$ oscillation with bound neutrons in Super-Kamiokande, with the full dataset from its first four run periods, representing an exposure of $0.37\mathrm{Mton}\text{−}\mathrm{years}$. The search used a multivariate analysis trained on simulated $n\to \overline{n}$ 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 $3.6\times {10}^{32}\mathrm{years}$ and on the neutron-antineutron oscillation time of ${\tau }_{n\to \overline{n}}>4.7\times {10}^8\mathrm{s}$ at 90% C.L.

• Received 6 December 2020
• Accepted 24 December 2020

DOI:https://doi.org/10.1103/PhysRevD.103.012008

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Published by the American Physical Society