D3 instantons in Calabi-Yau orientifolds with(out) fluxes

TL;DR

This work analyzes nonperturbative superpotential corrections from D3-brane instantons in Calabi–Yau orientifolds with D7 branes, focusing on fermion zero-mode counting with and without background flux. It shows that background flux can modify the zero-mode structure, enabling D3-instanton contributions in cases where they would vanish in the flux-free setup, as demonstrated in a concrete $K3\times T^2/Z_2$ orientifold example; D3-D7 sector effects provide additional constraints and, in a special coincident D3/D7 configuration, link multi-instanton effects to four-dimensional field theory instantons. The paper also discusses D5-instanton contributions in Type I theory and how abelian D9 sectors can realize nonzero superpotentials via gaugino condensation. Together, these results advance understanding of how fluxes and D3/D7 sector dynamics shape nonperturbative superpotentials and moduli stabilization scenarios in string compactifications.

Abstract

We investigate the instanton effect due to D3 branes wrapping a four-cycle in a Calabi-Yau orientifold with D7 branes. We study the condition for the nonzero superpotentials from the D3 instantons. For that matter we work out the zero mode structures of D3 branes wrapping a four-cycle both in the presence of the fluxes and in the absence of the fluxes. In the presence of the fluxes, the condition for the nonzero superpotential could be different from that without the fluxes. We explicitly work out a simple example of the orientifold of $K3 \times T^2/Z_2$ with a suitable flux to show such behavior. The effects of D3-D7 sectors are interesting and give further constraints for the nonzero superpotential. In a special configuration where D3 branes and D7 branes wrap the same four-cycle, multi-instanton calculus of D3 branes could be reduced to that of a suitable field theory. The structure of D5 instantons in Type I theory is briefly discussed.