Notes on unoriented D-brane instantons

TL;DR

The notes survey unoriented D-brane instantons, classifying them into gauge and exotic types and detailing how they produce non-perturbative effects across string backgrounds. By connecting worldsheet and D-brane perspectives, the authors derive ADHM-driven instanton constructions from branes within branes, enumerate vertex operators, and study orbifold projections like $T^6/Z_3$ and $T^4/Z_2$ to illustrate non-perturbative superpotentials and four-hyperini amplitudes. Through Heterotic–Type I duality, they compute and match worldsheet and ED-string corrections, revealing how instantons generate moduli-dependent masses and couplings, and how zero-mode counting governs the presence or absence of superpotential terms. The discussion highlights threshold corrections, moduli stabilization routes, and the rich interplay between unoriented instantons, dualities, and non-perturbative dynamics in compactifications, pointing to ongoing developments in localization and wall-crossing. Overall, the work provides a detailed framework for understanding non-perturbative effects from unoriented D-brane instantons and their phenomenological implications in string compactifications.

Abstract

In the first lecture, we discuss basic aspects of worldsheet and penta-brane instantons as well as (unoriented) D-brane instantons, which is our main focus here, and threshold corrections to BPS-saturated couplings. The second lecture is devoted to non-perturbative superpotentials generated by `gauge' and `exotic' instantons living on D3-branes at orientifold singularities. In the third lecture we discuss the interplay between worldsheet and D-string instantons on $T^4/Z_2$. We focus on a 4-fermi amplitude, give Heterotic and perturbative Type I descriptions, and offer a multi D-string instanton interpretation. We conclude with possible interesting developments.