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

TL;DR

This study searches for neutron-antineutron oscillations in Super-Kamiokande data, setting new lower limits on the oscillation lifetime, thereby testing baryon number violation with a large exposure and advanced analysis techniques.

Contribution

It provides the first comprehensive search using the full Super-Kamiokande data set with multivariate analysis, establishing the most stringent limits to date on neutron-antineutron oscillation.

Findings

No significant excess of candidate events was observed.

Lower limit on neutron-antineutron oscillation time is 4.7×10^8 seconds.

Lower limit on $ar n$ appearance lifetime in $^{16}$O is 3.6×10^32 years.

Abstract

As a baryon number violating process with $\Delta B=2$, neutron-antineutron oscillation ($n\to\bar n$) provides a unique test of baryon number conservation. We have performed a search for $n\to\bar n$ 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 $n\to\bar n$ 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 $3.6\times{10}^{32}$ years and on the neutron-antineutron oscillation time of $\tau_{n\to\bar n} > 4.7\times10^{8}$~s at 90\% C.L..