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 {C} {C}{}erenkov-equivalent energy and consistent with atmospheric neutrino interactions are identified. Of these, 610 have a single {C} {C}{}erenkov 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 ν_μ). In the momentum range 100<pₑ<1500 MeV/c and 300<p_μ<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.