  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
$\theta_{23}\leq \pi/4$ yields a best-fit mixing angle
$\sin^2(2\theta_{23})=1.000$ and mass splitting $|\Delta m^2_{32}| =2.44 \times
10^{-3}$ eV$^2$/c$^4$. If $\theta_{23}\geq \pi/4$ is assumed, the best-fit
mixing angle changes to $\sin^2(2\theta_{23})=0.999$ and the mass splitting
remains unchanged.
