New results on atmospheric neutrinos from Soudan 2

TL;DR

This paper reports updated measurements of atmospheric neutrino interactions from Soudan-2, providing evidence for nu_mu to nu_tau oscillations and constraining oscillation parameters using detailed event analysis.

Contribution

First detailed analysis of atmospheric neutrino oscillations using Soudan-2 data with improved L/E reconstruction and parameter constraints.

Findings

Measured R = 0.68 ± 0.11 ± 0.06 indicating muon neutrino deficit.

Constrained oscillation parameters to 0.46 < sin^2 2θ < 1.0 and 2.2×10^-4 < Δm^2 < 2.2×10^-2 eV^2.

Compatible distribution of partially contained events supports nu_mu to nu_tau oscillation scenario.

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 nu_mu/nu_e ratio-of-ratios and find R = 0.68 \pm 0.11 \pm 0.06. Assuming this anomalously low R-value is the result of nu_mu flavor disappearance via nu_mu to nu_tau oscillation, we select samples of charged current events which offer good resolution, event-by-event, for L/Enu reconstruction. Oscillation-weighted Monte Carlo events are fitted to these data events using a chisq function summed over bins of log(L/E_nu). The region allowed in the (sin^2 2\theta, \Delta m^2) plane at 90% CL is obtained using the Feldman-Cousins procedure: 0.46 < sin^2 2\theta < 1.0 and 2.2x10^-4 < \Delta m^2 < 2.2x10^-2 ev^2. A small but relatively energetic sample of partially contained nu_mu events has also been isolated. Their distribution in log(L/E_vis) relative to null oscillation Monte Carlo is compatible with nu_mu to nu_tau oscillation scenarios within the parameter region allowed by our contained events.