The Irreducible Axion Background

TL;DR

This paper demonstrates that the irreducible relic density of axions from freeze-in production imposes strong constraints on their properties, surpassing previous bounds and applicable to other particles like sterile neutrinos.

Contribution

It introduces a novel argument based on decay constraints to set stringent bounds on axion parameters across a wide mass range, surpassing existing experimental limits.

Findings

Constraints on axion-photon coupling at 10 keV: g_{aγγ} ≲ 8.1×10^{-14} GeV^{-1}

Constraints on axion-electron coupling at 100 keV: g_{aee} ≲ 8.0×10^{-15}

Method applicable to other particles like sterile neutrinos.

Abstract

Searches for dark matter decaying into photons constrain its lifetime to be many orders of magnitude larger than the age of the Universe. A corollary statement is that the abundance of any particle that can decay into photons over cosmological timescales is constrained to be much smaller than the cold dark-matter density. We show that an $\textit{irreducible}$ freeze-in contribution to the relic density of axions is in violation of that statement in a large portion of the parameter space. This allows us to set stringent constraints on axions in the mass range $100\rm \;eV - 100\; MeV$. At $10\rm \; keV$ our constraint on a photophilic axion is $g_{a\gamma \gamma} \lesssim 8.1 \times 10^{-14}~{\rm GeV}^{-1}$, almost three orders of magnitude stronger than the bounds established using horizontal branch stars; at $100~{\rm keV}$ our constraint on a photophobic axion coupled to electrons is $g_{aee} \lesssim 8.0 \times 10^{-15}$, almost four orders of magnitude stronger than present results. Although we focus on axions, our argument is more general and can be extended to, for instance, sterile neutrinos.