New results on atmospheric neutrinos from Soudan 2

Abstract

Neutrino interactions recorded in a 5.1 fiducial kiloton-year exposure of the Soudan-2 iron tracking calorimeter are analyzed for effects of neutrino oscillations. Using contained single track and single shower events, we update our measurement of the atmospheric ν μ ν e ratio-of-ratios and find R = 0.68 ± 0.11 ± 0.06. Assuming this anomalously low R-value is the result of ν μ flavor disappearance via ν μ to ν τ oscillation, we select samples of charged current events which offer good resolution, event-by-event, for L E ν reconstruction. Oscillation-weighted Monte Carlo events are fitted to these data events using a χ 2 function summed over bins of log ( L E ν ). The region allowed in the (sin 2 2θ, Δ m 2) plane at 90% CL is obtained using the Feldman-Cousins procedure: 0.46 < sin 2 θ ≤ 1.0 and 2.2 × 10 −4 < Δ m 2 < 2.2 × 10 −2 eV 2. A small but relatively energetic sample of partially contained ν μ events has also been isolated. Their distribution in log( L E vis ) relative to null oscillation Monte Carlo is compatible with ν μ to ν τ oscillation scenarios within the parameters region allowed by our contained events.