17-keV neutrino, MSW mechanism, and supernova constraints

A simple form for the neutrino mass matrix describing ${\nu }_e$, ${\nu }_{\mu }$, ${\nu }_{\tau }$, and a sterile state ${\nu }_s$ is proposed which accommodates the 17-keV neutrino as a ${\nu }_{\mu }-{\nu }_{\tau }$ pseudo Dirac pair and simultaneously resolves the solar-neutrino puzzle via ${\nu }_e-{\nu }_s$ Mikheyev-Smirnov-Wolfenstein (MSW) oscillation. This model, which is a specific realization of a scheme proposed recently by Caldwell and Langacker, is automatically free of all supernova constraints. It is shown that the mass matrix follows in a technically natural manner in extensions of the standard model with spontaneously broken global ${\mathrm{U}\mathrm{}\left(1\right)\mathrm{}}_{L_e-L_{\mu }}\times {\mathrm{U}\mathrm{}\left(1\right)\mathrm{}}_{L_{\tau }}$ symmetry. The 17-keV neutrino decays to ${\nu }_e$ and a Majoron with a lifetime of order ${10}^{-1\mathrm{}}$-${10}^{-2\mathrm{}}$ sec, thus satisfying all cosmological and astrophysical constraints.