Measurement of neutrino oscillation by the K2K experiment

We present measurements of ν_μ disappearance in K2K, the KEK to Kamioka long-baseline neutrino oscillation experiment. One hundred and twelve beam-originated neutrino events are observed in the fiducial volume of Super-Kamiokande with an expectation of 158.1^+9.2<sub-8.6 events without oscillation. A distortion of the energy spectrum is also seen in 58 single-ring muon-like events with reconstructed energies. The probability that the observations are explained by the expectation for no neutrino oscillation is 0.0015% (4.3σ). In a two flavor oscillation scenario, the allowed Δm² region at sin² 2θ = 1 is between 1.9 and 3.5 × 10⁻³ eV² at the 90% C.L. with a best-fit value of 2.8 × 10⁻³ eV².