Hierarchical neutrino masses and mixing in nonminimal SU(5)

We consider the problem of neutrino masses and mixing within the framework of a nonminimal supersymmetric SU(5) model extended by adding a set of 1, 24 chiral superfields accommodating three right-handed neutrinos. A type I + III seesaw mechanism can then be realized, giving rise to a hierarchical mass spectrum for the light neutrinos of the form m(3) > m(2) >> m(1), consistent with present data. The extra colored states are pushed to the unification scale by proton stability constraints, while the intermediate seesaw energy scale and the unification scale are maintained in phenomenologically acceptable ranges. We also examine the issue of the large neutrino mixing hierarchy theta(23) > theta(12) >> theta(13) in the above framework of hierarchical neutrino masses.