D-branes and textures

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.