Non-perturbative Yukawa Couplings from String Instantons

TL;DR

This work shows that non-perturbative string effects from Euclidean D2-brane instantons can generate crucial Yukawa couplings and mass terms that are perturbatively absent in Type II orientifold constructions with D-branes. By analyzing an $E2$-instanton with $O(1)$ gauge group, the authors demonstrate how the perturbatively forbidden SU(5) Yukawa coupling $\langle 10\,10\,5_H\rangle$ can arise with a factorized structure $Y^{\alpha} Y^{\beta}$ and an exponential suppression $e^{-S_{E2}}$, potentially explaining generation hierarchies in flipped $SU(5)$. They further present a globally consistent model in which instantons generate mass terms for exotic matter, with a factorized mass matrix $M^{I,J}_{A,B} = h^I_A h^J_B$ and a calculable prefactor $C' M_s e^{-S_{E2}}$, illustrating a mechanism to decouple unwanted states. Overall, the results highlight the vital role of stringy instantons in realizing phenomenologically viable couplings and mass terms in D-brane GUTs, connecting perturbative string constructions to non-perturbative dynamics and potential M-theory dual pictures.

Abstract

Non-perturbative D-brane instantons can generate perturbatively absent though phenomenologically relevant couplings for Type II orientifold compactifications with D-branes. We discuss the generation of the perturbatively vanishing SU(5) GUT Yukawa coupling of type 10 10 5_H. Moreover, for a simple globally consistent intersecting D6-brane model, we discuss the generation of mass terms for matter fields. This can serve as a mechanism for decoupling exotic matter.

Figures (2)

• Figure 1: Absorption of zero modes
• Figure 2: Absorption of the zero modes