QCD Axion Strings or Seeds?

TL;DR

This paper investigates how QCD axion strings influence electroweak symmetry breaking, especially during phase transitions, by using an effective theory approach to understand their role as seeds or catalysts in the process.

Contribution

It introduces an effective-theory framework to analyze the impact of axion strings on electroweak phase transitions, highlighting conditions where they significantly alter the dynamics.

Findings

Axion strings can act as seeds for bubble nucleation during first order phase transitions.

The impact of axion strings depends on the portal coupling strength between Higgs and PQ sectors.

Strings can trigger classical instabilities that complete the electroweak phase transition.

Abstract

We study the impact of QCD axion strings in the cosmological history of electroweak (EW) symmetry breaking, focussing on the minimal KSVZ axion model. We consider the case of the pure SM Higgs potential as well as a simple scenario with a first order EW phase transition. We capture the effect of the Peccei-Quinn (PQ) sector within an effective-theory approach for the Higgs field, where the axion string core and the heavy PQ states are integrated out. The relevant parameters in this effective theory are controlled by the size of the portal coupling between the Higgs and the PQ scalar, and the mass of the PQ radial excitation. We determine the range of portal couplings for which the axion strings can strongly affect the dynamics of EW symmetry breaking. In the case of a first order EW phase transition, the strings can act as seeds by either catalyzing the nucleation of (non-spherical) bubbles, or leading to the completion of the phase transition by triggering a classical instability.