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

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.

Comments: 6 pages, v2: references and text updated to match journal version