Measurements of atmospheric neutrinos and antineutrinos in the MINOS Far Detector
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 $\barν_μ$ interactions, and 701 contained-vertex showers, composed mainly of charged-current $ν_e$ and $\barν_e$ 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 $\barν_μ$ events. The observed ratio of $\barν_μ$ to $ν_μ$ events is compared with the Monte Carlo simulation, giving a double ratio of $R^data_\barν/ν/R^MC_\barν/ν = 1.03 ± 0.08 (stat.) ± 0.08 (syst.)$. The $ν_μ$ and $\barν_μ$ 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 $|Δ m^2| = (1.9 ± 0.4) × 10^-3 eV^2$ and $sin^2 2θ > 0.86$. The fit is extended to incorporate separate $ν_μ$ and $\barν_μ$ oscillation parameters, returning 90% confidence limits of $|Δ m^2|-|Δ \barm^2| = 0.6^+2.4_-0.8 × 10^-3 eV^2$ on the difference between the squared-mass splittings for neutrinos and antineutrinos.