Precise Measurement of the Neutrino Mixing Parameter θ23 from Muon Neutrino Disappearance in an Off-Axis Beam

New data from the T2K neutrino oscillation experiment produce the most precise measurement of the neutrino mixing parameter θ₂₃. Using an off-axis neutrino beam with a peak energy of 0.6 GeV and a data set corresponding to 6.57×10²⁰ protons on target, T2K has fit the energy-dependent ν_μ oscillation probability to determine oscillation parameters. The 68% confidence limit on sin²(θ₂₃) is 0.514-0.056+0.055 (0.511±0.055), assuming normal (inverted) mass hierarchy. The best-fit mass-squared splitting for normal hierarchy is Δm322=(2.51±0.10)×10^-3 eV2/c4 (inverted hierarchy: Δm132=(2.48±0.10)×10^-3 eV2/c4). 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.