On the predictivity of axion dark matter in the presence of Peccei-Quinn breaking

TL;DR

This paper explores how small explicit Peccei-Quinn symmetry breaking before the QCD transition affects axion dark matter predictions, potentially altering relic abundance calculations and extending the viable parameter space.

Contribution

It demonstrates that explicit symmetry breaking can significantly modify axion relic abundance predictions, challenging the assumption of a unique dark matter prediction.

Findings

Early-time dynamics depend on the symmetry-breaking scale μ.

Relic abundance can be influenced by parameters beyond the decay constant f_a.

The parameter space for axion dark matter can be extended due to symmetry breaking effects.

Abstract

It is shown that the post-inflationary quantum chromodynamics (QCD) axion need not lead to a unique one-parameter prediction for the dark matter abundance whenever small explicit Peccei-Quinn symmetry breaking becomes dynamically relevant before the QCD transition. Although strongly constrained by the strong CP bound, such breaking remains phenomenologically viable and introduces a mass scale $\mu$ that can control the early-time dynamics, as the QCD contribution to the axion mass is thermally suppressed at high temperatures. In this case, the axion string-wall network annihilates earlier, and the relic abundance is no longer primarily set by QCD dynamics alone, but instead depends on $\mu$, in addition to $f_a$, the axion decay constant. This effect overlaps with the parameter space relevant for QCD axion dark matter and, depending on ultraviolet parameters and initial conditions, can extend across it entirely.