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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 × 1020protons on target, T2K has fit the energy-dependent νμoscillation 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) × 10-3eV2/c4(inverted hierarchy: Δm13= (2.48 ± 0.10) × 10-3eV2/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.

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Physical Review Letters