D-branes and textures

Abstract

We examine the flavor structure of the trilinear superpotential couplings which can result from embedding the Standard Model within D-brane sectors in type-IIB orientifold models, which are examples within the type-I string framework. We find in general that the allowed flavor structures of the Yukawa coupling matrices to leading order are given by basic variations on the ``democratic" texture ansatz. In certain interesting cases, the Yukawa couplings have a novel structure in which a single right-handed fermion couples democratically at leading order to three left-handed fermions. We discuss the viability of such a ``single right-handed democracy'' in detail; remarkably, even though there are large mixing angles in the u,d sectors separately, the CKM mixing angles are small. The analysis demonstrates the ways in which the type-I superstring framework can provide a rich setting for investigating novel resolutions to the flavor puzzle.