Neutrino-induced upward stopping muons in Super-Kamiokande

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.)×10−13 cm−2s−1sr−1 compared to an expected flux of 0.73±0.16(theo.)×10−13 cm−2s−1sr−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 sin22θ>0.7 and 1.5×10−3<Δm2<1.5×10−2 eV2 at 90% confidence level, providing a confirmation of the observation of neutrino oscillations by Super-Kamiokande using the contained atmospheric neutrino events.