Model of Four Light Neutrinos in the Light of All Present Data

Motivated by existing and recent data on possible neutrino oscillations, we propose a model of four light neutrinos (${\nu }_e,{\nu }_{\mu },{\nu }_{\tau }$, and a singlet ${\nu }_S$) with a pattern of masses and mixing derivable from a $Z_5$ discrete symmetry and the seesaw mechanism. Atmospheric neutrino oscillations occur between ${\nu }_{\mu }$ and ${\nu }_{\tau }$ as pseudo-Dirac partners; whereas solar neutrino oscillations occur between ${\nu }_e$ and ${\nu }_S$, a linear combination of which is massless. Additional oscillations may occur between ${\nu }_e$ and ${\nu }_{\mu }$ to account for the recent observation of the LSND (Liquid Scintillator Neutrino Detector) experiment.