Cosmological Implications of Axion-Matter Couplings

TL;DR

This paper revisits cosmological constraints on axion-like particles' couplings to matter, refining production rate calculations and highlighting their complementarity to astrophysical bounds, especially through effects on the universe's radiation density.

Contribution

It provides updated, more precise calculations of axion-matter thermalization rates and clarifies the relationship between cosmological and astrophysical constraints on these particles.

Findings

Cosmological bounds on axion couplings are tighter with refined calculations.

Constraints from cosmology are complementary to supernova bounds.

Thermalization below the electroweak scale impacts the effective number of relativistic species.

Abstract

Axions and other light particles appear ubiquitously in physics beyond the Standard Model, with a variety of possible couplings to ordinary matter. Cosmology offers a unique probe of these particles as they can thermalize in the hot environment of the early universe for any such coupling. For sub-MeV particles, their entropy must leave a measurable cosmological signal, usually via the effective number of relativistic particles, $N_\mathrm{eff}$. In this paper, we will revisit the cosmological constraints on the couplings of axions and other pseudo-Nambu-Goldstone bosons to Standard Model fermions from thermalization below the electroweak scale, where these couplings are marginal and give contributions to the radiation density of $\Delta N_\mathrm{eff} > 0.027$. We update the calculation of the production rates to eliminate unnecessary approximations and find that the cosmological bounds on these interactions are complementary to astrophysical constraints, e.g. from supernova SN 1987A. We additionally provide quantitative explanations for these bounds and their relationship.