K2K Results

The K2K, the KEK to Kamioka long-baseline neutrino oscillation experiment, observes neutrino oscillation; a reduction of muon neutrinos together with a distortion of the energy spectrum. One hundred and seven beam-originated neutrino events are observed in Super-Kamiokande in expectation of 151−10+12 events without oscillation. The spectrum distortion by oscillation is also seen in fifty-seven single-ring muon-like events usable for energy reconstruction. The probability that the observations are explained by statistical fluctuation with no neutrino oscillation is 0.005 % (4.0σ). In a two flavor oscillation scenario, the allowed Δm2 region at sin22θ=1 is between 1.9 and 3.6 × 10−3 eV2 at the 90 % C.L. with the central value of 2.8 × 10−3 eV2. The result of K2K confirms neutrino oscillation of atmospheric neutrinos reported by Super-Kamiokande collaboration.