K2K Experiment

The K2K, the KEK to Kamioka long baseline neutrino oscillation experiment, observes evidences of neutrino oscillation: a reduction of muon neutrinos together with a distortion of the energy spectrum. Fifty six beam-originated neutrino events are observed in Super-Kamiokande with an expectation of 80.1-5.4+6.2 events. Among those events, twenty-nine single-ring muon-like events are used to reconstruct the neutrino energy. The measured energy spectrum better matches with the expected one with oscillation. The probability of no oscillation is calculated to be less than 1 %. In a two flavor oscillation scenario, the allowed Δm2 region at sin22θ = 1 is between 1.5 and 3.9 x 10-3 eV2 at the 90 % C.L. The result at K2K is consistent with the oscillation of atmospheric neutrinos reported by Super-Kamiokande collaboration