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 \ensuremath{\sim}50 MeV of \ifmmode \check{C}\else \v{C}\fi{}erenkov-equivalent energy and consistent with atmospheric neutrino interactions are identified. Of these, 610 have a single \ifmmode \check{C}\else \v{C}\fi{}erenkov ring. Single-ring interactions are efficiently separated into those containing a showering particle (produced mainly by ${\ensuremath{\nu}}_{e}$) and those containing a nonshowering particle (produced mainly by ${\ensuremath{\nu}}_{\ensuremath{\mu}}$). In the momentum range $100<{p}_{e}<1500$ MeV/c and $300<{p}_{\ensuremath{\mu}}<1500$ MeV/c, the fraction of nonshowering events is 0.36\ifmmode\pm\else\textpm\fi{}0.02(stat)\ifmmode\pm\else\textpm\fi{}0.02(syst). Based on detailed models of neutrino production and interaction, a fraction of 0.51\ifmmode\pm\else\textpm\fi{}0.01(stat)\ifmmode\pm\else\textpm\fi{}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\ifmmode\pm\else\textpm\fi{}2(stat)% of events are accompanied by one or more muon decays, while 43\ifmmode\pm\else\textpm\fi{}1(stat)% are expected. Further studies that could reduce systematic errors and discover the cause of these discrepancies are suggested.