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_-8.6+9.2 events without oscillation. A distortion of the energy spectrum is also seen in 58 single-ring muonlike 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 sin²2θ=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.