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 ν_μ→ν_τ oscillations is sin²(2θ₂₃)=0.98 and |Δm₃₂²|=2.65×10^-3 eV². The boundary of the 90% confidence region includes the points (sin²(2θ₂₃),|Δm₃₂²|)=(1.0,3.1×10^-3 eV²), (0.84, 2.65×10^-3 eV²) and (1.0, 2.2×10^-3 eV²).