Observation of Atmospheric Neutrino Oscillations in Soudan 2

A deficit of atmospheric nu(mu) events, consistent with the hypothesis of neutrino oscillations, is observed in the 5.90 kiloton-year fiducial exposure of the Soudan 2 detector. An unbinned maximum likelihood analysis of the neutrino L/E distribution has been carried out using the Feldman-Cousins prescription. The probability of the no oscillation hypothesis is 5.8x10(-4). The 90% confidence allowed region in the sin(2)2theta,Deltam(2) plane is presented. The region includes the 90% confidence allowed region of the Super-K experiment.