Heavy QCD axion model in light of pulsar timing arrays

TL;DR

This paper proposes that a heavy QCD axion model with a supercooled first-order phase transition can explain the nanohertz gravitational wave background observed by pulsar timing arrays.

Contribution

It introduces a heavy QCD axion model with a supercooled phase transition at the TeV scale as a source of gravitational waves matching recent observations.

Findings

A supercooled phase transition can produce gravitational waves in the nanohertz range.

The model links the Peccei-Quinn symmetry breaking to observable gravitational wave signals.

Reheating temperature at the GeV scale is consistent with the phase transition dynamics.

Abstract

Recently, pulsar timing array experiments reported the observation of a stochastic gravitational wave background in the nanohertz range frequency band. We show that such a signal can be originated from a cosmological first-order phase transition (PT) within a well-motivated heavy (visible) QCD axion model. Considering the Peccei-Quinn symmetry breaking at the TeV scale in the scenario, we find a supercooled PT, in the parameter space of the model, prolonging the PT with the reheating temperature at the GeV scale.