THE SIMPLEST MODEL FOR THE 17 KEV NEUTRINO AND THE MSW EFFECT

The recently reported spectral anomalies in tritium, S-35 and C-14 beta decays and the observed solar neutrino deficit may be simply understood in a model without heavy leptons. We present a simple SU (2) x U (1) model where all neutrino masses arise radiatively. Two 17 keV states form the quasi-Dirac tau neutrino nu-s that decays invisibly as nu-s --> nu-e,mu + majoron with a lifetime that can be shorter than approximately 10(8) s. The solar neutrino data can be understood through matter-enhanced nu-e --> nu-mu oscillations. Improved nu-e disappearance experiments as well as nu-e,mu --> nu-tau-oscillation searches can test the model. The splitting of the quasi-Dirac neutrino can be revealed not only through the process nu-tau --> nu-sterile but also in the conventional nu-e,mu --> nu-tau-oscillations.