ATMOSPHERIC NEUTRINO EVENT RATES - THE EXPECTATIONS

Naturally occurring energetic neutrinos produced by interactions of cosmic rays in the atmosphere produce a signal in deep underground detectors which is a convolution of neutrino flux, neutrino cross section and detection efficiency. The predicted ratio of events induced by v(e) as compared to v(mu) is relatively robust (because many sources of uncertainty cancel in the ratio), but it differs significantly from what is observed. Interpretations that involve new physics (e.g. neutrino oscillations or nucleon decay) have been proposed. One interpretation in terms of neutrino oscillations would imply a low value of neutrino-induced upward muons. Although there is no strong evidence for such a deficit, uncertainties in calculating the expected absolute rate prevent one from eliminating this interpretation at present. Precise measurement of muon fluxes at high altitude, as well as calibration of detectors with neutrino beams from accelerators, will help clarify the situation.