Stable non-BPS states in string theory: a pedagogical review

TL;DR

This paper provides a detailed pedagogical review of stable non-BPS states in string theory, focusing on the stable non-BPS D-particle in Type I theory and its origin from tachyon condensation in D1/anti-D1 pairs.

Contribution

It offers a comprehensive explanation of the formation and stability of non-BPS states, especially the non-BPS D-particle, using duality and conformal field theory analysis.

Findings

Stable non-BPS D-particle predicted by heterotic/Type I duality.

Originates from tachyon condensation in D1/anti-D1 pairs.

Exact conformal field theory description provided.

Abstract

We present a pedagogical review of the stable non-BPS states in string theory which have recently attracted some attention in the literature. In particular, following the analysis of A. Sen, we discuss in detail the case of the stable non-BPS D-particle of Type I theory whose existence is predicted (and required) by the heterotic/Type I duality. We show that this D-particle originates from an unstable bound state formed by a D1/anti-D1 pair of Type IIB in which the tachyon field acquires a solitonic kink configuration. The mechanism of tachyon condensation is discussed first at a qualitative level and then with an exact conformal field theory analysis.