Open Strings and Heterotic Instantons

TL;DR

This paper explores non-perturbative effects in heterotic string theories, proposing heterotic D-instantons and wrapped non-BPS D-strings as sources, and discusses implications for the necessity of open strings in all closed string theories.

Contribution

It introduces heterotic D-instantons and wrapped non-BPS D-strings as non-perturbative effects, highlighting their dependence on worldsheet and spacetime interactions and implications for open string necessity.

Findings

Heterotic D-instantons arise from heterotic disk diagrams.

Wrapped Euclidean non-BPS D-strings contribute to non-perturbative effects.

All closed string theories may inherently include open strings.

Abstract

Motivated by closed string perturbation theory arguments by S. Shenker, we consider non-perturbative effects of characteristic strength $\mathcal{O}(e^{-1/g_{s}})$, with $g_{s}$ the closed string coupling constant, in supersymmetric critical heterotic string theories. We argue that in 10D such effects arise from heterotic "D-instantons," i.e. heterotic disk diagrams, whose existence relies on a non-trivial interplay between worldsheet and spacetime degrees of freedom. In compactifications of the $\mathrm{SemiSpin}(32)$ heterotic string, we argue that similar effects can arise from wrapped Euclidean non-BPS "D-strings." Two general principles arise: The first is that the consistency of those heterotic branes on which the fundamental string can end relies on an inflow mechanism for spacetime degrees of freedom. The second is that Shenker's argument, taken to its logical conclusion, implies that all closed string theories must exhibit open strings as well.