Fermion masses and mixing from an extra gauge symmetry

Abstract

Assuming that a horizontal abelian (gauge) symmetry is at the origin of texture zeros in the fermion mass matrices we show how realistic mass patterns can be generated in the presence of scalar fields whose vacuum expectation value breaks the extra $U(1)$ symmetry. In the simplest scenario with just one pair of singlet fields and under the assumption of L-R symmetry one obtains quark mass matrices {\it à la Fritzsch}. The $U(1)$ symmetry can be made anomaly free by the Green-Schwarz mechanism in which case the canonical unification of the gauge couplings emerges as its byproduct. The generation of neutrino masses requires either two extra heavy scalar (Higgs) fields to determine the texture structure of the righthanded neutrino mass matrix or the latter will contain a hierarchy of scales.