Measurement of neutrino oscillation by the K2K experiment

Abstract

We present measurements of ${\nu }_{\mu }$ 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% ($4.3\sigma$). In a two-flavor oscillation scenario, the allowed $\Delta m^2$ region at ${sin}^{2}2\theta =1$ is between 1.9 and $3.5\times {10}^{-3}{\mathrm{eV}}^2$ at the 90% C.L. with a best-fit value of $2.8\times {10}^{-3}{\mathrm{eV}}^2$.