Predictive Grand Unified Textures for Quark and Neutrino Masses and Mixings

We propose new textures for the fermion Yukawa matrices which are generalizations of the so-called Stech ansatz. We discuss how these textures can be realized in supersymmetric grand unified models with horizontal symmetry $SU(3)_H$ among the fermion generations. In this framework the mass and mixing hierarchy of fermions (including neutrinos) can emerge in a natural way. We emphasize the central role played by the $SU(3)_H$ adjoint Higgs field which reduces $SU(3)_H$ to $U(2)_H$ at the GUT scale. A $SO(10)\times SU(3)_H$ model is presented in which the desired Yukawa textures can be obtained. The phenomenological implications of these textures are thoroughly investigated. Among various realistic possibilities for the Clebsch factors between the quark and lepton entries, we find three different solutions which provide excellent fits of the quark masses and CKM mixing angles. Interestingly, all these solutions predict the correct amount of CP violation via the CKM mechanism, and, in addition, lead to an appealing pattern of the neutrino masses and mixing angles. In particular, they all predict nearly maximal 23 mixing and small 12 mixing in the lepton sector, respectively in the range needed for the explanation of the atmospheric and solar neutrino anomaly.