Closed strings from nothing

TL;DR

This paper investigates the fate of open strings on unstable D-branes, proposing a solitonic flux tube model that suggests open strings can form closed string-like objects through confinement and strong coupling effects.

Contribution

It introduces a novel solitonic flux tube model of open strings that can mimic closed strings, challenging previous ideas about open string disappearance at tachyon minimum.

Findings

Flux tube solutions with fundamental string charge

Large charge flux tubes have light open strings in the core

Confinement makes open string degrees of freedom heavy and strongly coupled

Abstract

We study the physics of open strings in bosonic and type II string theories in the presence of unstable D-branes. When the potential energy of the open string tachyon is at its minimum, Sen has argued that only closed strings remain in the perturbative spectrum. We explore the scenario of Yi and of Bergman, Hori and Yi, who argue that the open string degrees of freedom are strongly coupled and disappear through confinement. We discuss arguments using open string field theory and worldsheet boundary RG flows, which seem to indicate otherwise. We then describe a solitonic excitation of the open string tachyon and gauge field with the charge and tension of a fundamental closed string. This requires a double scaling limit where the tachyon is taken to its minimal value and the electric field is taken to its maximum value. The resulting flux tube has an unconstrained spatial profile; and for large fundamental string charge, it appears to have light, weakly coupled open strings living in the core. We argue that the flux tube acquires a size or order $\alpha'$ through sigma model and string coupling effects; and we argue that confinement effects make the light degrees of freedom heavy and strongly interacting.