Seesaw-model predictions for theτ-neutrino mass

The observed deficit of solar neutrinos cannot be explained by a cooler solar model, which would suppress high-energy neutrinos more than low-energy neutrinos, contrary to observation. The small neutrino masses and flavor mixing implied by matter-amplified neutrino oscillations in the Sun are most naturally interpreted in terms of minimal grand unification theories (GUT's) incorporating the seesaw mechanism. In two such theories, SO(10) GUT and supersymmetric GUT, that are consistent with all laboratory experiments, the neutrino mixing is like quark Cabibbo-Kobayashi-Maskawa mixing and the neutrino masses are proportional to the squares of the up-quark masses. For the SO(10) GUT model, the symmetry-breaking scale is intermediate and the $\mu$-neutrino mass is close to that observed in solar-neutrino oscillations. Although the seesaw-model mass predictions are less reliable than the mixing-angle predictions, the $\tau$-neutrino mass may lie in the cosmologically important range 4-28 eV and be accessible to laboratory neutrino oscillation experiments or to observation in a nearby supernova.