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

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 nn occurrences to have ≥ 4 prongs (tracks and showers) and to have kinematics compatible with nN annihilation within a nucleus. We observe five candidate events, with an estimated background from atmospheric neutrino and cosmic ray induced events of 4.5 ± 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% CL for the nn oscillation time in iron: TA(Fe) > 7.2×1031 years. The corresponding lower limit for oscillation of free neutrons is τnn > 1.3× 10 seconds. PACS numbers: 11.30.Fs, 12.20.Fv, 12.60.Jv, 14.20.Dh