Measurement of Neutrino Oscillation Parameters from Muon Neutrino Disappearance with an Off-axis Beam

Abstract

The T2K collaboration reports a precision measurement of muon neutrino disappearance with an off-axis neutrino beam with a peak energy of 0.6 GeV. Near detector measurements are used to constrain the neutrino flux and cross section parameters. The Super-Kamiokande far detector, which is 295 km downstream of the neutrino production target, collected data corresponding to $3.01 \times 10^{20}$ protons on target. In the absence of neutrino oscillations, $205 \pm 17$ (syst.) events are expected to be detected and only 58 muon neutrino event candidates are observed. A fit to the neutrino rate and energy spectrum assuming three neutrino flavors, normal mass hierarchy and $θ_{23}\leq π/4$ yields a best-fit mixing angle $\sin^2(2θ_{23})=1.000$ and mass splitting $|Δm^2_{32}| =2.44 \times 10^{-3}$ eV$^2$/c$^4$. If $θ_{23}\geq π/4$ is assumed, the best-fit mixing angle changes to $\sin^2(2θ_{23})=0.999$ and the mass splitting remains unchanged.

8 pages, 4 figures, revised version accepted for publication in Physical Review Letters