Measurement of Neutrino Oscillation by the K2K Experiment

TL;DR

The K2K experiment measured muon neutrino disappearance over a long baseline, providing strong evidence for neutrino oscillation with specific parameters, confirming the phenomenon beyond previous experiments.

Contribution

This study provides the first direct measurement of muon neutrino disappearance in a long-baseline experiment, establishing key oscillation parameters with high statistical significance.

Findings

Observed 112 events vs. 158 expected without oscillation

Detected energy spectrum distortion consistent with oscillation

Determined oscillation parameters with Delta m^2 between 1.9 and 3.5 x 10^{-3} eV^2

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^{+9.2}_{-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% (4.3sigma). In a two flavor oscillation scenario, the allowed Delta m^2 region at sin^2(2theta) is between 1.9 and 3.5 x 10^{-3} eV^2 at the 90% C.L. with a best-fit value of 2.8 x 10^{-3} eV^2.