Search for neutron-antineutron oscillations using multiprong events in Soudan 2

Abstract

We have searched for neutron-antineutron oscillations using the 5.56 fiducial kiloton-year exposure of the Soudan 2 iron tracking calorimeter. We require candidate $n\mathrm{n\ifmmode \bar{}\else \={}\fi{}}$ occurrences to have $>~4$ prongs (tracks and showers) and to have kinematics compatible with $\mathrm{n\ifmmode \bar{}\else \={}\fi{}}N$ annihilation within a nucleus. We observe five candidate events, with an estimated background from atmospheric neutrino and cosmic ray induced events of $4.5\ifmmode\pm\else\textpm\fi{}1.2$ events. Previous experiments with smaller exposures observed no candidates, with estimated background rates similar to this experiment. We set a lifetime lower limit at 90% C.L. for the $n\mathrm{n\ifmmode \bar{}\else \={}\fi{}}$ oscillation time in iron: ${T}_{A}(\mathrm{Fe})>7.2\ifmmode\times\else\texttimes\fi{}{10}^{31} \mathrm{yr}.$ The corresponding lower limit for oscillation of free neutrons is ${\ensuremath{\tau}}_{n\mathrm{n\ifmmode \bar{}\else \={}\fi{}}}>1.3\ifmmode\times\else\texttimes\fi{}{10}^{8} \mathrm{sec}.$