Fate of the sterile neutrino

In light of recent Super-Kamiokande data and global fits that seem to exclu de both pure nu(mu) --> nu(s) oscillations of atmospheric neutrinos and pur e nu(e) --> nu(s) oscillations of solar neutrinos (where nu(s) is a sterile neutrino), we reconsider four-neutrino models to explain the LSND, atmosph eric, and solar neutrino oscillation indications. We argue that the solar d ata, 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 separ ated by order 1 eV, the day-night asymmetry, seasonal dependence, and energ y dependence for B-8 neutrinos should be small. Alternatively, we find that four-neutrino models with one mass eigenstate widely separated from the ot hers (and with small sterile mixings to active neutrinos) may now be accept able in light of recently updated LSND results; the Cl-37 data can be accom modated in this model. For each scenario, we present simple four-neutrino m ixing matrices that fit the stated criterion and discuss future tests. (C) 2000 Elsevier Science B.V. All rights reserved.