  New data from the T2K neutrino oscillation experiment produce the most
precise measurement of the neutrino mixing parameter theta_{23}. Using an
off-axis neutrino beam with a peak energy of 0.6 GeV and a data set
corresponding to 6.57 x 10^{20} protons on target, T2K has fit the
energy-dependent nu_mu oscillation probability to determine oscillation
parameters. Marginalizing over the values of other oscillation parameters
yields sin^2 (theta_{23}) = 0.514 +0.055/-0.056 (0.511 +- 0.055), assuming
normal (inverted) mass hierarchy. The best-fit mass-squared splitting for
normal hierarchy is Delta m^2_{32} = (2.51 +- 0.10) x 10^{-3} eV^2/c^4
(inverted hierarchy: Delta m^2_{13} = (2.48 +- 0.10) x 10^{-3} eV^2/c^4).
Adding a model of multinucleon interactions that affect neutrino energy
reconstruction is found to produce only small biases in neutrino oscillation
parameter extraction at current levels of statistical uncertainty.
