(Non-)BPS bound states and D-brane instantons

TL;DR

This paper investigates how non-invariant D-brane instantons contribute to the superpotential in 4D N=1 orientifold compactifications, especially through multi-instanton processes and bound states across stability walls.

Contribution

It provides a microscopic analysis of non-invariant D-brane instantons and their role in superpotential generation via multi-instanton effects and bound states.

Findings

Non-invariant instantons can generate superpotential terms through multi-instanton processes.

Bound states of D-branes can decay or form depending on the moduli space region.

Microscopic analysis shows BPS states can compensate for decay, enabling superpotential contributions.

Abstract

We study non-perturbative effects in four-dimensional N=1 supersymmetric orientifold compactifications due to D-brane instantons which are generically not invariant under the orientifold projection. We show that they can yield superpotential contributions via a multi-instanton process at threshold. Some constituents of this configuration form bound states away from the wall of marginal stability which can decay in other regions of moduli space. A microscopic analysis reveals how contributions to the superpotential are possible when new BPS states compensate for their decay. We study this concretely for D2-brane instantons along decaying special Lagrangians in Type IIA and for D5-branes instantons carrying holomorphic bundles in Type I theory.