Indications of Neutrino Oscillation in a 250 km Long-baseline Experiment

TL;DR

The K2K long-baseline experiment provides evidence for neutrino oscillation through observed flux reduction and spectral distortion in muon neutrinos over 250 km, supporting the oscillation hypothesis.

Contribution

This study presents the first indication of neutrino oscillation in a long-baseline experiment using a controlled neutrino beam and large-scale detector.

Findings

Observed 56 neutrino events versus 80 expected without oscillation.

Energy spectrum distortion consistent with oscillation models.

Probability of fluctuation without oscillation is less than 1%.

Abstract

The K2K experiment observes indications of neutrino oscillation: a reduction of $\nu_\mu$ flux together with a distortion of the energy spectrum. Fifty-six beam neutrino events are observed in Super-Kamiokande (SK), 250 km from the neutrino production point, with an expectation of $80.1^{+6.2}_{-5.4}$. Twenty-nine one ring $\mu$-like events are used to reconstruct the neutrino energy spectrum, which is better matched to the expected spectrum with neutrino oscillation than without. The probability that the observed flux at SK is explained by statistical fluctuation without neutrino oscillation is less than 1%.