Measurements of atmospheric neutrinos and antineutrinos in the MINOS far detector

Abstract

This paper reports measurements of atmospheric neutrino and antineutrino interactions in the MINOS Far Detector, based on 2553 live-days (37.9 kton-years) of data. A total of 2072 candidate events are observed. These are separated into 905 contained-vertex muons and 466 neutrino-induced rock-muons, both produced by charged-current $ν_μ$ and $\overline{ν}_μ$ interactions, and 701 contained-vertex showers, composed mainly of charged-current $ν_e$ and $\overline{ν}_e$ interactions and neutral-current interactions. The curvature of muon tracks in the magnetic field of the MINOS Far Detector is used to select separate samples of $ν_μ$ and $\overline{ν}_μ$ events. The observed ratio of $\overline{ν}_μ$ to $ν_μ$ events is compared with the Monte Carlo (MC) simulation, giving a double ratio of $R^{data}_{\bar ν/ν}/R^{MC}_{\bar ν/ν}=1.03±0.08(\text{stat})±0.08(\text{syst})$. The $ν_μ$ and $\overline{ν}_μ$ data are separated into bins of L/E resolution, based on the reconstructed energy and direction of each event, and a maximum likelihood fit to the observed $L/E$ distributions is used to determine the atmospheric neutrino oscillation parameters. This fit returns 90% confidence limits of $|Δm^2|=(1.9±0.4)×10^{−3} \text{eV}^2$ and $\text{sin}^2 2θ>0.86$. The fit is extended to incorporate separate $ν_μ$ and $\overline{ν}_μ$ oscillation parameters, returning 90% confidence limits of $|Δm^2|−|Δ\overline{m}^2|=0.6^{+2.4}_{−0.8}×10^{−3} \text{eV}^2$ on the difference between the squared-mass splittings for neutrinos and antineutrinos.