Large Lepton Mixing in Seesaw Models - Coset-space Family Unification -

Abstract

We show that the large mixing between $\nu_\mu$ and $\nu_\tau$ observed by the SuperKamiokande collaboration is a quite natural prediction in a large class of seesaw models. This large mixing is basically due to the unparallel family structure suggested from the observed mass hierarchies in quark and lepton mass matrices. We show that the unparallel family structure is automatically realized in ``coset-space family unification'' model based on $E_7/SU(5) \times U(1)^3$. This model also suggests the small angle MSW solution to the solar neutrino problem.