S-duality and Tensionless 5-branes in Compactified Heterotic String Theory

TL;DR

This paper proves S-duality in heterotic string theory compactified on T^6, calculates tensions of BPS 5-branes, and explores stability and moduli space of Kaluza-Klein monopoles and NS5-branes.

Contribution

It provides a simple proof of S-duality, computes tensions of specific branes, and analyzes stability and moduli spaces in heterotic string compactifications.

Findings

Kaluza-Klein monopole becomes tensionless at string-length radius

NS5-brane stability depends on circle radius

Moduli space of two Kaluza-Klein monopoles matches heterotic A_1 singularity

Abstract

We give a simple proof of the known S-duality of Heterotic String theory compactified on a T^6. Using this S-duality we calculate the tensions for a class of BPS 5-branes in Heterotic String theory on a circle. One of these, the Kaluza-Klein monopole, becomes tensionless when the radius of the circle is equal to the string length. We study the question of stability of the Heterotic NS5-brane with a transverse circle. For large radii the NS5-brane is absolutely stable. However for small radii it is only marginally stable. We also study the moduli space of 2 Kaluza-Klein monopoles and show that it is equal to the moduli space of a Heterotic A_1 singularity.