First muon-neutrino disappearance study with an off-axis beam

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 νμ→ντ oscillations is sin⁡2(2θ23)=0.98 and |Δm322|=2.65×10-3  eV2. The boundary of the 90% confidence region includes the points (sin⁡2(2θ23),|Δm322|)=(1.0,3.1×10-3  eV2), (0.84, 2.65×10-3  eV2) and (1.0, 2.2×10-3  eV2).