Cosmological Implications of Axion-Matter Couplings

Understand

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 $\Delta N_\mathrm{eff} > 0.027$.