Axion strings from string axions

TL;DR

This paper explores axion strings originating from string theory in warped extra dimensions, highlighting their formation during phase transitions and their potential role in dark matter relic abundance, with implications for future simulations.

Contribution

It introduces a string-theoretic model of axion strings formed during phase transitions, distinct from traditional field-theoretic strings, and discusses their unique properties and formation mechanisms.

Findings

Axion strings form at bubble junctions during phase transitions.

Their tensions are lower than typical field-theoretic axion strings.

Simulation studies are needed to understand their impact on relic abundance.

Abstract

A favored scenario for axions to be dark matter is for them to form a cosmic string network that subsequently decays, allowing for a tight link between the axion mass and relic abundance. We discuss an example in which the axion is protected from quantum gravity effects that would spoil its ability to solve the strong CP problem: namely a string theoretic axion arising from gauge symmetry in warped extra dimensions. Axion strings arise following the first-order Randall-Sundrum compactification phase transition, forming at the junctions of three bubbles during percolation. Their tensions are at the low scale associated with the warp factor, and are parametrically smaller than the usual field-theory axion strings, relative to the scale of their decay constant. Simulations of string network formation by this mechanism must be carried out to see whether the axion mass-relic density relation depends on the new parameters in the theory.