Electron- and muon-neutrino content of the atmospheric flux

Neutrino interactions from a 7.7 kton yr exposure of the IMB-3 detector are analyzed. A total of 935 contained events radiating over ∼50 MeV of Cerenkov-equivalent energy and consistent with atmospheric neutrino interactions are identified. Of these, 610 have a single Cerenkov ring. Single-ring interactions are efficiently separated into those containing a showering particle (produced mainly by νe) and those containing a nonshowering particle (produced mainly by νμ). In the momentum range 100<pe<1500 MeV/c and 300<pμ<1500 MeV/c, the fraction of nonshowering events is 0.36±0.02(stat)±0.02(syst). Based on detailed models of neutrino production and interaction, a fraction of 0.51±0.01(stat)±0.05(syst) is expected. This deficit of nonshowering, or excess of showering, events relative to the total is supported by an independent analysis of muon decay signals. In the same sample 33±2(stat)% of events are accompanied by one or more muon decays, while 43±1(stat)% are expected. Further studies that could reduce systematic errors and discover the cause of these discrepancies are suggested