Using $\Delta N_{\rm eff}$ to constrain preferred axion model dark matter

TL;DR

This paper explores how measurements of the effective number of neutrino species, ΔN_eff, can constrain preferred axion dark matter models by analyzing their impact on dark radiation and the early Universe's thermal history.

Contribution

It demonstrates that decay of heavy quarks in preferred axion models can produce detectable dark radiation, allowing constraints from cosmological observations.

Findings

Existing constraints exclude 40% of the parameter space for preferred axion models.

Decays of heavy quarks after axion decoupling contribute to dark radiation.

Planck CMB measurements are sensitive to these effects.

Abstract

Preferred axion models are minimal realizations of the Peccei-Quinn solution to the strong CP problem while providing a dark matter candidate. These models invoke new heavy quarks that interact strongly with the Standard Model bringing them into thermal equilibrium in the early Universe. We show that for a number of these models, the heavy quarks will decay after axions have decoupled from the Standard Model thermal bath. As a consequence, any axion products in the decay form a component of dark radiation. This provides the potential to differentiate between preferred axion models through measurements of the number of relativistic degrees of freedom. The most sensitive of which comes from the Planck collaboration's measurements of the Cosmic Microwave Background. We find that existing constraints allow us to rule out regions of parameter space for 40% of the canonical preferred axion models.