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

The T2K collaboration reports a precision measurement of muon neutrinodisappearance with an off-axis neutrinobeam with a peak energy of 0.6 GeV. Near detector measurements are used to constrain the neutrinoflux and cross section parameters. The Super-Kamiokandefar detector, which is 295 km downstream of the neutrinoproduction target, collected data corresponding to 3.01×1020 protons on target. In the absence of neutrino oscillations, 205±17 (syst.) events are expected to be detected and only 58 muon neutrinoevent candidates are observed. A fit to the neutrinorate and energy spectrum assuming three neutrinoflavors, normal mass hierarchy and θ23≤π/4 yields a best-fit mixing angle sin2(2θ23)=1.000 and mass splitting |Δm232|=2.44×10−3 eV2/c4. If θ23≥π/4 is assumed, the best-fit mixing angle changes to sin2(2θ23)=0.999 and the mass splitting remains unchanged.