Impact of First-order Electroweak Phase Transition on QCD Axion

TL;DR

This paper proposes a new mechanism called recurrent misalignment that extends the viable parameter space for the QCD axion across a wide range of decay constants by involving a first-order electroweak phase transition, with implications for detection and gravitational waves.

Contribution

It introduces a novel axion dynamics mechanism across the electroweak phase transition, broadening the viable axion parameter space without fine-tuning.

Findings

Extends axion decay constant range to [10^8, 10^14] GeV

Predicts detectable gravitational waves from the scenario

Enhances prospects for axion detection

Abstract

The QCD axion addresses the strong CP problem and dark matter via the misalignment mechanism, typically requiring a decay constant $f_a\sim \mathcal{O}(10^{12}$ GeV), unless the initial misalignment angle ($\theta_i$) is fine-tuned. This work presents a novel approach where the possibility that the QCD axion satisfying the correct relic is extended over a broad range for $f_a\in [10^8, 10^{14}$] GeV without fine-tuning the $\theta_i$, by introducing a new phase of axion oscillation dynamics across the electroweak phase transition (EWPT). This mechanism, we call it {\it{recurrent ~misalignment}}, is a result of a non-renormalizable Peccei-Quinn symmetry breaking interaction involving the axion and the sector responsible for making the EWPT of first order. The scenario not only enhances the QCD axion parameter space in terms of its detection possibility, but also provides a unique probe by detectable gravitational waves.