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 × 1020 protons on target. In the absence of neutrino oscillations, 205 ± 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 θ23≤ π/4 yields a best-fit mixing angle sin2(2θ23)=1.000 and mass splitting |Δ m232| =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.
