Unoriented D-brane Instantons vs Heterotic worldsheet Instantons

TL;DR

The paper addresses nonperturbative corrections to four-hyperini Fermi couplings in a six-dimensional Type I / Heterotic dual pair on $T^4/\mathbb{Z}_2$. It develops a detailed duality-based computation: the heterotic side gives a worldsheet instanton sum decomposed into trivial, degenerate, and non-degenerate ${\Gamma_2}$ orbits, while the Type I side yields perturbative disk contributions for special fixed-point configurations and nonperturbative ED1-instanton corrections that map to the heterotic orbit sums. A central result is the explicit matching between degenerate heterotic orbifold contributions and Type I disk amplitudes, together with a corresponding mapping of non-degenerate (worldsheet) instantons to ED1 instantons via a 2D gauge-theory and symmetric-product CFT analysis. The work provides a concrete dictionary for multi-instanton effects in theories with eight supercharges and suggests pathways to extend the duality dictionary to four dimensions and moduli-stabilization scenarios via anomalous U(1) dynamics.

Abstract

We discuss Fermi interactions of four hyperini generated by ``stringy'' instantons in a Type I / Heterotic dual pair on T^4/Z_2.