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

A. Lerda, R. Russo

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.

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

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.

Paper Structure

This paper contains 8 sections, 139 equations, 1 figure, 3 tables.

Table of Contents

  1. Introduction
  2. The SO(32) heterotic string and the Type I theory: a brief perturbative description
  3. The SO(32) heterotic string
  4. The Type I string
  5. The Type I theory and its D1 brane
  6. A Non-BPS D-particle in the Type I theory
  7. Conformal field theory at the critical radius
  8. The Type I D-particle in Minkowski space

Figures (1)

  • Figure 1: This figure resumes the relations between different D-branes in type-II superstring theories. The squares represent the usual supersymmetric BPS D-branes, while the circles stay for the non-BPS configurations (unstable in Type II theories). Starting from a pair formed by a Dp and an anti-Dp brane, a non-BPS brane can be constructed in two ways: one can mod out the system by $(-1)^{F_L}$ (horizontal arrows) or condense the tachyon living on its world-volume (vertical arrows). By repeating these operations twice, one finds a supersymmetric configuration. The diagonal links represent the usual T-duality.