A Measurement of Atmospheric Neutrino Flux Consistent with Tau Neutrino Appearance

A search for the appearance of tau neutrinos from \mutau oscillations in the atmospheric neutrinos has been performed using the atmospheric neutrino data from the Super-Kamiokande-I experiment. A tau neutrino enriched sample is selected by a statistical analysis method with a set of variables characterizing the decay of tau leptons. The zenith angle distribution of the selected sample is fitted with a combination of the expected tau neutrino signals resulting from oscillations and the predicted atmospheric neutrino background events including oscillations. The Super-Kamiokande-I atmospheric neutrino data for 1489.2 days, which find a best fit tau neutrino appearance signal of 138 $\pm$ 48 (stat.) $^{+15}_{-32}$ (sys.), disfavor the hypothesis of no tau neutrino appearance by 2.4 sigma. The data are consistent with tau neutrino appearance.