On the (Non) Perturbative Origin of Quark Masses in D-brane GUT Models

Abstract

We examine the issue of generating the perturbatively absent $10\cdot 10\cdot 5^H$ SU(5)/flipped SU(5)Yukawa couplings in type II D-brane orientifold compactifications of string theory both at the perturbative (PER) and the non-perturbative (NP) level. We find at the PER level, higher order terms like $10\cdot 10\cdot {\bar{5}}^H\cdot {\bar{5}}^H\cdot {\bar{5}}^H\cdot {\bar{5}}^H\cdot 1^H\cdot 1^H$ in SU(5) may be responsible for the relevant quark mass generation in models with general intersecting D6-branes. Euclidean D2-brane instantons on the other hand can also generate at the NP via the term $10\cdot 10\cdot {\bar{5}}^H\cdot {\bar{5}}^H\cdot {\bar{5}}^H\cdot {\bar{5}}^H$ the relevant quark masses by the use of just the $U(1{)}_b$ brane, for SU(5) and flipped SU(5) GUTS classes of models. We provide local examples of rigid O(1) instantons within the $T^6/{\mathbb{Z}}_2\times {\mathbb{Z}}_2^{\prime }$ toroidal orientifold with torsion, whose NP contribution to the masses gets minimal as it is induced by just a duplicated disk diagram.

Comments: 26 pages, 1 figure; v2: typos corrected, comments added