Gravitational Waves from Fundamental Axion Dynamics

TL;DR

This paper explores a fundamental axion model where PQ symmetry breaking via the Coleman-Weinberg mechanism predicts gravitational waves detectable by future experiments, with a highly constrained parameter space.

Contribution

It introduces a highly predictive, asymptotically free QCD axion model with a PQ phase transition that produces observable gravitational waves.

Findings

PQ phase transition is strongly first order

Gravitational wave spectrum depends on PQ scale and QCD coupling

Model predicts gravitational waves within future detector sensitivities

Abstract

A totally asymptotically free QCD axion model, where all couplings flow to zero in the infinite energy limit, was recently formulated. A very interesting feature of this fundamental theory is the ability to predict some low-energy observables, like the masses of the extra fermions and scalars. Here we find and investigate a region of the parameter space where the Peccei-Quinn (PQ) symmetry is broken quantum mechanically through the Coleman-Weinberg mechanism. This results in an even more predictive framework: the axion sector features only two independent parameters (the PQ symmetry breaking scale and the QCD gauge coupling). In particular, we show that the PQ phase transition is strongly first order and can produce gravitational waves within the reach of future detectors. The predictivity of the model leads to a rigid dependence of the phase transition (like its duration and the nucleation temperature) and the gravitational wave spectrum on the PQ symmetry breaking scale and the QCD gauge coupling.