YUKAWA TEXTURES IN STRING UNIFIED MODELS WITH SU(4)CIRCLE-TIMES-O(4) SYMMETRY

We discuss the origin of Yukawa textures in the string-inspired and st ring-derived models based on the gauge group SU(4) x SU(2)(L) x SU(2)( R) supplemented by a U(1)(X) gauged family symmetry. The gauge symmetr ies are broken down to those of the minimal supersymmetric standard mo del which is the effective theory below 10(16) GeV. The combination of the U(1)(X) family symmetry and the Pati-Salam gauge group leads to a successful and predictive set of Yukawa textures involving two kinds of texture zeros: horizontal and vertical texture zeros. We discuss bo th symmetric and nonsymmetric textures in models of this kind, and in the second case perform a detailed numerical fit to the charged fermio n mass and mixing data. Two of the Yukawa textures allow a low energy fit to the data with a total chi(2) of 0.39 and 1.02, respectively, fo r three degrees of freedom. We also make a first attempt at deriving t he nonrenormalizable operators required for the Yukawa textures from s tring theory.