Neutrino-induced upward stopping muons in Super-Kamiokande

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.) ×10 −13 cm −2 s −1 sr −1 compared to an expected flux of 0.73±0.16 (theo.) ×10 −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 θ >0.7 and 1.5×10 −3 Δm 2 −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.