Resurrecting Low-Mass Axion Dark Matter Via a Dynamical QCD Scale

TL;DR

This paper explores how a dynamical QCD scale can alter the evolution of the axion field, potentially reviving low-mass axion dark matter models by suppressing relic abundance and expanding viable parameter space.

Contribution

It introduces a framework where the dynamical evolution of the QCD scale affects axion cosmology, revealing new possibilities for axion dark matter with higher Peccei-Quinn scales.

Findings

Multiple phases of axion damping and oscillation occur due to the evolving QCD scale.

The relic abundance of axions can be significantly suppressed.

This suppression allows for lighter axions with larger Peccei-Quinn scales to be viable dark matter candidates.

Abstract

In the framework where the strong coupling is dynamical, the QCD sector may confine at a much higher temperature than it would in the Standard Model, and the temperature-dependent mass of the QCD axion evolves in a non-trivial way. We find that, depending on the evolution of $\Lambda_{\mathrm{QCD}}$, the axion field may undergo multiple distinct phases of damping and oscillation leading generically to a suppression of its relic abundance. Such a suppression could therefore open up a wide range of parameter space, resurrecting in particular axion dark-matter models with a large Peccei-Quinn scale $f_a\gg 10^{12}~\mathrm{GeV}$, i.e., with a lighter mass than the standard QCD axion.