String Decomposition and Gravitational Waves in High-quality Axion Gauge Theories

TL;DR

This paper investigates high-quality axion gauge theories, showing how axion string decomposition can resolve cosmological issues and predicts gravitational wave signatures detectable by future experiments.

Contribution

It introduces a mechanism for decomposing axion strings in Barr-Seckel models, ensuring viable cosmology and linking string decay to gravitational wave signals.

Findings

Decomposition of axion strings into unit-winding and gauge strings.

Timely destruction of axion string-wall networks.

Prediction of a double-plateau gravitational wave spectrum.

Abstract

The QCD axion can successfully solve the strong CP problem under the condition that the Peccei-Quinn symmetry is respected to extremely high standard. We explore a class of gauge theories that accommodate a high-quality axion, known as the Barr-Seckel models, paying special attention to the cosmology of topological defects. In models with domain wall number equal to unity, we show that axion strings with winding number larger than one can always be decomposed into a number of unit-winding axion strings and pure gauge strings. This mechanism enables the axion string-wall network to be destroyed timely to render a viable cosmology. The subsequent decay of gauge strings into gravitational waves generically produces a double-plateau in frequency space allowing the mechanism to be tested by upcoming experiments.