Neutrino-induced upward stopping muons in Super-Kamiokande

TL;DR

This paper reports the observation of fewer upward stopping muons than expected in Super-Kamiokande, providing evidence for neutrino oscillations with specific parameters, confirmed through flux ratio analysis and zenith angle distribution fitting.

Contribution

It presents the first measurement of upward stopping muons and confirms neutrino oscillations using these events, complementing previous contained neutrino observations.

Findings

Measured muon flux is lower than theoretical predictions.

The flux ratio R is significantly smaller than expected.

Results support neutrino oscillation hypothesis with specific parameters.

Abstract

A total of 137 upward stopping muons of minimum energy 1.6 GeV are observed by Super-Kamiokande during 516 detector live days. The measured muon flux is 0.39+/-0.04(stat.)+/-0.02(syst.)x10^{-13}cm^{-2}s^{-1}sr^{-1} compared to an expected flux of 0.73+/-0.16(theo.)x10^{-13}cm^{-2}s^{-1}sr^{-1}. Using our previously-published measurement of the upward through-going muon flux, we calculate the stopping/through-going flux ratio R}, which has less theoretical uncertainty. The measured value of R=0.22+/-0.02(stat.)+/-0.01(syst.) is significantly smaller than the value 0.37^{+0.05}_{-0.04}(theo.) expected using the best theoretical information (the probability that the measured R is a statistical fluctuation below the expected value is 0.39%). A simultaneous fitting to zenith angle distributions of upward stopping and through-going muons gives a result which is consistent with the hypothesis of neutrino oscillations with the parameters sin^2 2\theta >0.7 and 1.5x10^{-3} < \Delta m^2 < 1.5x10^{-2} eV^2 at 90% confidence level, providing a confirmation of the observation of neutrino oscillations by Super-Kamiokande using the contained atmospheric neutrino events.