Quadratic Coupling of the Axion to Photons

TL;DR

This paper demonstrates that QCD axions couple to photons at one loop, leading to potential variations in the fine structure constant, which imposes constraints on axion dark matter models and extends to axion-like particles.

Contribution

It reveals a one-loop coupling between QCD axions and photons, and derives new constraints on axion parameter space from electromagnetic variation limits.

Findings

Axions induce temporal variations in the fine structure constant.

Constraints on axion parameter space are recast based on these variations.

The findings extend to axion-like particles, broadening the scope of the constraints.

Abstract

We show that the QCD axion couples to the electromagnetic kinetic term at one loop. The result is that if axions make up dark matter, they induce temporal variation of the fine structure constant $\alpha$, which is severely constrained. We recast these constraints on the QCD axion parameter space. We also discuss how to generalise our finding to axion-like particles, and the resulting constraints.