BPS Spectra and Non--Perturbative Couplings in N=2,4 Supersymmetric String Theories

TL;DR

This paper investigates the BPS spectrum and non-perturbative couplings in N=2,4 heterotic string theories, highlighting the role of intermediate states and proposing an S-T exchange symmetry to compute non-perturbative effects.

Contribution

It introduces the concept of intermediate BPS states in N=4 theories and explores their significance for S-T exchange symmetry in N=2 models, providing exact 1-loop correction results.

Findings

Identification of intermediate BPS states in N=4 theories.

Proposal of S-T exchange symmetry in N=2 models.

Exact computation of 1-loop gravitational couplings.

Abstract

We study the BPS spectrum in $D=4, N=4$ heterotic string compactifications, with some emphasis on intermediate $N=4$ BPS states. These intermediate states, which can become short in $N=2$ compactifications, are crucial for establishing an $S-T$ exchange symmetry in $N=2$ compactifications. We discuss the implications of a possible $S-T$ exchange symmetry for the $N=2$ BPS spectrum. Then we present the exact result for the 1-loop corrections to gravitational couplings in one of the heterotic $N=2$ models recently discussed by Harvey and Moore. We conjecture this model to have an $S-T$ exchange symmetry. This exchange symmetry can then be used to evaluate non-perturbative corrections to gravitational couplings in some of the non-perturbative regions (chambers) in this particular model and also in other heterotic models.