Experimental study of the atmospheric [Nu][mu]/[Nu]e ratio in the multi-GeV energy range

Abstract

The atmospheric neutrino flux ratio Nu$\sb-$mu/Nu$\sb-$e and its zenith angle dependence have been measured in the multi-GeV energy range using an exposure of 33.0 kiloton-years of the Super-Kamiokande detector. By comparing the data to a detailed Monte Carlo simulation, the ratio of the measured (Nu$\sb-$mu/Nu$\sb-$e) to the simulated value is 0.65 $\pm$ 0.05(stat.) $\pm$0.07(syst.). In addition, a strong distortion in the shape of the event zenith angle distribution was observed. The ratio of the number of upward to downward muon-like events was found to be 0.61 $\pm$ 0.05(stat.) $\pm$0.02(syst.) with an expected value of 0.98 $\pm$ 0.03(stat.) $\pm$0.02(syst.). The same ratio for e-like events was consistent with unity. These data provide strong evidence for muon neutrino to tau neutrino flavor oscillations with mass difference squared between $3\times 10\sp{-2}$ and $3\times 10\sp{-4}$ eV$\sp2$ and maximal mixing. These results are fully consistent with Super-Kamiokande results obtained at sub-GeV energies and also consistent with previous measurements.