Search for $n-\overline{n}$ oscillation in Super-Kamiokande

A search for neutron-antineutron ($n-\overline{n}$) oscillation was undertaken in Super-Kamiokande using the 1489 live-day or $2.45\times {10}^{34}$ neutron-year exposure data. This process violates both baryon and baryon minus lepton numbers by an absolute value of two units and is predicted by a large class of hypothetical models where the seesaw mechanism is incorporated to explain the observed tiny neutrino masses and the matter-antimatter asymmetry in the Universe. No evidence for $n-\overline{n}$ oscillation was found; the lower limit of the lifetime for neutrons bound in ${}^{16}\mathrm{O}$, in an analysis that included all of the significant sources of experimental uncertainties, was determined to be $1.9\times {10}^{32}\mathrm{years}$ at the 90% confidence level. The corresponding lower limit for the oscillation time of free neutrons was calculated to be $2.7\times {10}^8\mathrm{s}$ using a theoretical value of the nuclear suppression factor of $0.517\times {10}^{23}{\mathrm{s}}^{-1}$ and its uncertainty.

Figure 1

The distributions of the kinematic variables subject to additional criteria at each of the reduction steps described in the text including (a) number of rings, (b) visible energy, (c) total momentum, and (d) invariant mass. Circles indicate data points with statistical error bars; solid blue lines and dashed red lines represent the atmospheric neutrino MC and the $n-\overline{n}$ MC, respectively. Light vertical lines with arrows indicate the event selection criteria.

Figure 2

Total momentum vs the invariant mass after applying the selection criteria (a)–(b) on the FC sample: (A) signal MC, (B) atmospheric neutrino MC, and (C) data. The boxed region in each panel shows the criterion (c)–(d) for the $n-\overline{n}$ oscillation signal.