Neutron-antineutron oscillation search at Super-Kamiokande

As a baryon number violating process with $\Delta B=\Delta (B-L)=2$, neutron-antineutron oscillation ($n\to\bar n$) provides an important candidate and a unique probe to the baryon asymmetry. We performed a search for $n\to\bar n$ oscillation with the Super-Kamiokande (SK) experiment. Full exposure data set of SK was analyzed using a multi-variate analysis based on kinematic variables and basic distributions from simulated $n\to\bar n$ signal events and atmospheric neutrino backgrounds. The expected number of background events is 9.3, based on which the sensitivity of nuclear lifetime is calculated as $4.3\times{10}^{32}$ years, significantly improved from the present best-limit $1.9\times{10}^{32}$ years from SK-I.