Right-handed-neutrino Majorana mass at the SUSY GUT scale and the solution of the solar-neutrino problem

In the SUSY GUT scenario, it is natural to assume the right-handed-neutrino Majorana-mass scale to be ${10}^{16}$ GeV. This will in principle lead, by the seesaw mechanism, to a ${\nu }_{\tau }$ mass of order $ m_t^2 / (10^{16}, {\rm GeV}) \sim 3 \times 10^{-3}, {\rm eV} $. This suggests that the solution of the solar-neutrino puzzle should be either the MSW effect in ${\nu }_e$–${\nu }_{\tau }$ oscillations, with $ m_{\nu_{\tau}}^2 \sim 10^{-5}, {\rm eV}^2 $,orlong-wavelength$\nu_e$--$\nu_{\mu}$oscillations,with$ m_{\nu_{\mu}}^2 \sim 10^{-10}, {\rm eV}^2 $. These solutions require unexpectedly large mixings of ${\nu }_e$ with ${\nu }_{\tau }$ and ${\nu }_{\mu }$, respectively. I suggest a variation of the Dimopoulos–Hall–Raby model for the fermion mass matrices which can accomodate such large mixings.