The Irreducible Axion Background

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 $\text{irreducible}$ 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 $100\mathrm{eV}-100\mathrm{MeV}$. At $10\mathrm{keV}$ our constraint on a photophilic axion is $g_{a\gamma \gamma }\lesssim 8.1\times 10^{-14}{\mathrm{GeV}}^{-1}$, almost three orders of magnitude stronger than the bounds established using horizontal branch stars; at $100\mathrm{keV}$ 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.

Comments: 7+14 pages, 2+9 figures; v2 additional references and updated to the latest INTEGRAL constraints