Comparison of nu(mu)<->nu(tau) and nu(mu)<->nu(s) oscillations as solutions of the atmospheric neutrino problem

The simplest explanation for the atmospheric neutrino anomaly is the transition of muon neutrinos and antineutrinos into states of different flavor. Data from the reactor experiment CHOOZ, and the measurement of the energy and angle distribution of electronlike events at Super-Kamiokande put strong constraints on nu(mu)-nu(e) mixing, but the transitions nu(mu) --> nu(tau) and nu(mu) --> nu(s) are viable possibilities. If the muon neutrinos are mixed with a singlet sterile state, the energy and zenith angle dependence of the oscillation probability is modified by matter effects that are important for E-nu/Delta m(2)greater than or similar to 10(3) GeV/eV(2). We confront the theoretical predictions with existing data on contained events and neutrino-induced upward-going muons. [S0556-2821(98)00319-1]