First Muon-Neutrino Disappearance Study with an Off-Axis Beam

With data corresponding to 1.43×10²⁰ protons on target, we observe 31 fully-contained single muon-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 nu_mu -> nu_tau oscillations is sin²(2 θ₂₃) = 0.98 and |Δm²₃₂| = 2.65×10^-3 eV². The boundary of the 90 % confidence region includes the points (sin²(2 θ₂₃),|Δm²₃₂|) = (1.0, 3.1 10^-3 eV²), (0.84, 2.65×10^-3 eV²) and (1.0, 2.2×10^-3 eV²).