  A neutrino mass-mixing scheme which explains qualitatively all present
evidence for neutrino mass (the solar and atmospheric neutrino deficits, LSND,
and hot dark matter), and also makes possible heavy-element nucleosynthesis by
supernovae, requires at least one light sterile neutrino. String-inspired
models with sub-millimeter extra dimensions provide naturally light sterile
neutrinos, as is needed to explain the solar nu_e deficit. This bulk sterile
neutrino provides a better fit to the solar data than conventional models by
having vacuum oscillations of the nu_e to its zero mode and MSW oscillations to
its first few Kaluza-Klein modes. While the prediction of the Super-Kamiokande
energy spectrum gives a fit probability of 73%, the superior energy resolution
of SNO's charged-current spectrum will determine whether this neutrino scheme
is correct and can demonstrate that an extra dimension of ~60 mu m exists.
Should this be the case, there are important implications for supernovae,
ultra-high-energy cosmic rays, double beta decay, and dark matter.
