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We report a measurement of muon-neutrino disappearance in the T2K experiment. The 295-km muon-neutrino beam from Tokai to Kamioka is the first implementation of the off-axis technique in a long-baseline neutrino oscillation experiment. With data corresponding to 1.43×1020 protons on target, we observe 31 fully-contained single μ-like ring events in Super-Kamiokande, compared with an expectation of 104±14(syst) events without neutrino oscillations. The best-fit point for two-flavor ν μ→ν τ oscillations is sin2(2θ 23)=0.98 and |Δm322|=2.65×10 -3eV2. The boundary of the 90% confidence region includes the points ( sin2(2θ 23), |Δm322|)=(1.0,3.1×10 -3eV2), (0.84, 2.65×10 -3eV2) and (1.0, 2.2×10 -3eV2). © 2012 American Physical Society.

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Physical Review D - Particles, Fields, Gravitation and Cosmology