Measurement of neutrino oscillation by the K2K experiment

TLDR

The K2K experiment studies muon‑neutrino disappearance over a 250‑km baseline from KEK to Kamioka. The study measures muon‑neutrino disappearance in the K2K long‑baseline experiment. Observations of 112 events versus an expectation of 158.1 without oscillation, a distorted energy spectrum, and a 4.3σ significance support neutrino oscillations with Δm² between 1.9 and 3.5 × 10⁻³ eV² (best fit 2.8 × 10⁻³ eV²).

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.