Study of the nu_<mu>-deficit in the atmospheric neutriono fluxes. (Study of the origin of neutrino mass.)

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In 1994, a beam experiment was carried out to study the efficiency of separating electrons and muons, and to study various detector effects related to the nu_<mu>-deficit in the atmospheric neutrino fluxes. The result from this experiment showed that the efficiency of separating electrons and muons was 98% as expected by our Monte Carlo simulation. This result suggests that the atmospheric nu_<mu>-deficit is probably related to a real physics such as neutrino oscillations.The total number of atmospheric neutrino events was nearly doubled during this research period. The observed (nu_<mu>/nu_e) ratio relative to the expected one, (nu_<mu>/nu_e) _<Data>/ (nu_<mu>/nu_e) _<MC>, was 0.60<plus-minus>0.055 (stat.)<plus-minus>0.05 (syst.) at the sub-Gev energy range. The statistical significance of the nu_<mu>-deficit was about 5sigma.We also studied the atmospheric neutrino fluxes in the multi-GeV energy range. We observed nu_<mu>-deficit in this energy range too. The observed zenith-angle dependence of the nu_<mu>/nu_e ratio in this energy range was not uniform and was consistent with the expectation from neutrino oscillations.From those studies, we concluded that neutrino oscillations might account for the atmospheric netrino data. The mass of either nu_<mu>mu or nutau might be about 0.1 eV.