Open and Closed Strings from Unstable D-branes

TL;DR

The work analyzes the tachyon effective action on unstable D-branes to understand how electric flux tubes relate to fundamental strings. It shows that flux tubes ending on a kink must reside in a finite-T core whose energetic cost enforces vanishing transverse area, casting doubt on the naive interpreting of $T=\infty$ flux tubes as fundamental strings and motivating a rolled-up-kink construction for string-like objects. BCFT corroborates the EFT findings, while a finite-core string solution demonstrates a viable open-string description of a fundamental string ending on a kink, with Nambu–Goto dynamics emerging in an appropriate limit. The paper further proposes that tachyon matter and these flux-tube configurations describe high-density closed-string states produced in D-brane decay, linking open-string EFT to closed-string dynamics in the weak-coupling regime and highlighting the role of higher-derivative corrections for precise localization.

Abstract

The tachyon effective field theory describing the dynamics of a non-BPS D-$p$-brane has electric flux tube solutions where the electric field is at its critical value and the tachyon is at its vacuum. It has been suggested that these solutions have the interpretation of fundamental strings. We show that in order that an electric flux tube can `end' on a kink solution representing a BPS D-$(p-1)$-brane, the electric flux must be embedded in a tubular region inside which the tachyon is finite rather than at its vacuum where it is infinite. Energetic consideration then forces the transverse `area' of this tube to vanish. We suggest a possible interpretation of the original electric flux tube solutions around the tachyon vacuum as well as of tachyon matter as system of closed strings at density far above the Hagedorn density.