Fate of the sterile neutrino

In light of recent Super-Kamiokande data and global fits that seem to exclude both pure nu(mu) --> nu(s) oscillations of atmospheric neutrinos and pure nu(e) --> nu(s) oscillations of solar neutrinos (where nu(s) is a sterile neutrino), we reconsider four-neutrino models to explain the LSND, atmospheric, and solar neutrino oscillation indications. We argue that the solar data, with the exception of the Cl-37 results, are suggestive of v(e) --> v(s) oscillations that average to a probability of approximately 1/2. In this interpretation, with two pairs of nearly degenerate mass eigenstates separated by order 1 eV, the day-night asymmetry, seasonal dependence, and energy dependence for B-8 neutrinos should be small. Alternatively, we find that four-neutrino models with one mass eigenstate widely separated from the others (and with small sterile mixings to active neutrinos) may now be acceptable in light of recently updated LSND results; the Cl-37 data can be accommodated in this model. For each scenario, we present simple four-neutrino mixing matrices that fit the stated criterion and discuss future tests. (C) 2000 Elsevier Science B.V. All rights reserved.