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 Cerenkov-equivalent energy and consistent with atmospheric neutrino interactions are identified. Of these, 610 have a single C Cerenkov ring. Single-ring interactions are efficiently separated into those containing a showering particle (produced mainly by ₄) and those containing a nonshowering particle (produced mainly by _). In the momentum range 100<p₄<1500 MeV/c and 300<p_<1500 MeV/c, the fraction of nonshowering events is 0. 360. 02 (stat) 0. 02 (syst). Based on detailed models of neutrino production and interaction, a fraction of 0. 510. 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 332 (stat) % of events are accompanied by one or more muon decays, while 431 (stat) % are expected. Further studies that could reduce systematic errors and discover the cause of these discrepancies are suggested.