Dark matter from an even lighter QCD axion: trapped misalignment

TL;DR

This paper proposes a new mechanism called trapped misalignment for ultra-light QCD axion dark matter, expanding the viable mass range and linking it to experimental searches like CASPEr-Electric.

Contribution

It introduces trapped misalignment as a novel mechanism for axion dark matter production, allowing for a broader mass spectrum including fuzzy dark matter.

Findings

Dark matter can be explained by an ultra-light QCD axion with trapped misalignment.

The mechanism involves two stages of oscillations, delaying true oscillations.

Parameter space is accessible to upcoming experimental searches.

Abstract

We show that dark matter can be accounted for by an axion that solves the strong CP problem, but is much lighter than usual due to a $Z_\mathcal{N}$ symmetry. The whole mass range from the canonical QCD axion down to the ultra-light regime is allowed, with $3\le\mathcal{N}\lesssim65$. This includes the first proposal of a "fuzzy dark matter" QCD axion with $m_a\sim10^{-22}$ eV. A novel misalignment mechanism occurs --{\it trapped misalignment}-- due to the peculiar temperature dependence of the $Z_{\mathcal{N}}$ axion potential. The dark matter relic density is enhanced because the axion field undergoes two stages of oscillations: it is first trapped in the wrong minimum, which effectively delays the onset of true oscillations. Trapped misalignment is more general than the setup discussed here, and may hold whenever an extra source of Peccei-Quinn breaking appears at high temperatures. Furthermore, it will be shown that trapped misalignment can dynamically source the recently proposed kinetic misalignment mechanism. All the parameter space is within tantalizing reach of the experimental projects for the next decades. For instance, even Phase I of CASPEr-Electric could discover this axion.