Aspects of Dirichlet S-branes

TL;DR

This paper explores the properties, stability, and potential physical relevance of Dirichlet S-branes in string theory, analyzing their boundary conditions, instabilities, and dualities with D-branes under various perturbations.

Contribution

It provides new insights into the instability mechanisms of S-branes, their T-duality relations, and the nature of their boundary states within type II string theories.

Findings

S-branes require fine-tuning to be extended space-like defects.

Under perturbations, S-branes tend to decay into D-brane / anti-D-brane pairs.

Boundary states of S-branes can involve imaginary R-R fields or real fields in type II* theory.

Abstract

In this note, we discuss some features of the Dirichlet S-brane, defined as a Dirichlet boundary condition on a time-like embedding coordinate of open strings. We analyze the Euclidean theory on the S-brane world-volume, and trace its instability to the infinite fine-tuning of the initial conditions required to produce an infinitely extended space-like defect. Using their equivalence under T-duality with D-branes with supercritical electric field, we argue that under generic perturbation, S-branes turn into D-brane / anti-D-branes. We extract the imaginary part of the cylinder amplitude, and interpret its inverse as a ``decay length'', beyond which a pair of S-branes annihilates. Finally, we reconsider the boundary state of the Dirichlet S-brane and find that it is either a solution of type II string theory with imaginary R-R fields, or a solution of type II$^*$ with real fields. This leaves the non-BPS S-branes as potentially physical solutions of type II string theory.