T2K reports its first measurements of the parameters governing the disappearance of ν_μ in an off-axis beam due to flavor change induced by neutrino oscillations. The quasimonochromatic ν_μ beam, produced with a peak energy of 0.6 GeV at J-PARC, is observed at the far detector Super-Kamiokande, 295 km away, where the ν_μ survival probability is expected to be minimal. Using a data set corresponding to 4.01×10²⁰ protons on target, 34 fully contained μ-like events were observed. The best-fit oscillation parameters are sin²(θ₂₃)=0.45 and |Δm̄₃₂²|=2.51×10^-3 eV² with 68% confidence intervals of $0.38--0.64$ and 2.26--2.80×10^-3 eV², respectively. These results are in agreement with existing antineutrino parameter measurements and also with the ν_μ disappearance parameters measured by T2K.
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Abe et al. (2016) studied this question.
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