Topological Defects and nano-Hz Gravitational Waves in Aligned Axion Models

TL;DR

This paper investigates the formation of topological defects in aligned axion models with multiple scalars, revealing their potential to produce nano-Hz gravitational waves detectable by pulsar timing arrays.

Contribution

It introduces a detailed analysis of string-wall networks in aligned axion models and predicts gravitational wave signals in the nano-Hz range, constraining axion decay constants.

Findings

String-wall networks can survive until the QCD phase transition.

Collapse of the network produces detectable gravitational waves.

Current pulsar timing constrains axion decay constants below ~100 TeV.

Abstract

We study the formation and evolution of topological defects in an aligned axion model with multiple Peccei-Quinn scalars, where the QCD axion is realized by a certain combination of the axions with decay constants much smaller than the conventional Peccei-Quinn breaking scale. When the underlying U(1) symmetries are spontaneously broken, the aligned structure in the axion field space exhibits itself as a complicated string-wall network in the real space. We find that the string-wall network likely survives until the QCD phase transition if the number of the Peccei-Quinn scalars is greater than two. The string-wall system collapses during the QCD phase transition, producing a significant amount of gravitational waves in the nano-Hz range at present. The typical decay constant is constrained to be below O(100) TeV by the pulsar timing observations, and the constraint will be improved by a factor of 2 in the future SKA observations.