Search for Neutron-Antineutron Oscillations Using Multiprong Events in Soudan 2

TL;DR

This study searches for neutron-antineutron oscillations using the Soudan 2 detector, setting new lower limits on the oscillation lifetime based on observed candidate events and background estimates.

Contribution

It provides the first search with a large exposure in Soudan 2 and establishes new lower bounds on neutron-antineutron oscillation times.

Findings

Observed five candidate events with an estimated background of 4.5 1.2.

Set a lower limit on oscillation time in iron: T_A(Fe) > 7.2x10^{31} years.

Lower limit for free neutron oscillation: _{n-nbar} > 1.3x10^8 seconds.

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-nbar occurrences to have .GE. 4 prongs (tracks and showers) and to have kinematics compatible with nbar-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 \pm 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 for oscillation time in iron: T_A(Fe) > 7.2x10^{31} years. The corresponding lower limit for oscillation of free neutrons is \tau_{n-nbar} > 1.3x10^8 seconds.