Dark Radiation Constraints on Heavy QCD Axions

TL;DR

This paper investigates constraints on heavy QCD axions from dark radiation measurements, computing their abundance, decay effects, and potential signals in future CMB experiments, considering various decay scenarios and mass ranges.

Contribution

It provides a comprehensive, model-independent analysis of heavy QCD axion abundance, decay effects, and their observable signatures in CMB measurements, including new numerical computations.

Findings

Heavy axions decaying before neutrino decoupling are constrained by dark radiation limits.

Predicted signals in CMB Stage 4 experiments for certain axion parameter regions.

Bounds on axions with mass below 1 MeV from their dark matter and dark radiation contributions.

Abstract

The naturalness problem of PQ symmetry motivates study of the heavy QCD axion, with masses $m_a >$ 1 MeV generated at scales above the QCD scale, and low values of the PQ symmetry breaking scale, $f_a$. We compute the abundance of such axions in a model-independent way, assuming only that they freeze-out after reheating from inflation, and are not subsequently diluted by new physics. If these axions decay between neutrino decoupling and the last scatter era of the Cosmic Microwave Background (CMB), they dilute the neutrinos and their abundance is constrained by CMB measurements of the energy density in dark radiation, $N_{\rm eff}$. We accurately compute this bound using a numerical code to evolve the axion momentum distribution, including many key processes and effects previously ignored. We assume that the only relevant axion decays are to final states involving Standard Model particles. We determine regions of $(m_a, f_a)$ that will give a signal in $N_{\rm eff}$ at CMB Stage 4 experiments. We similarly compute the $N_{\rm eff}$ bound and CMB Stage 4 signal for heavy axions that can decay to light mirror photons. Finally, we compute the bounds on heavy axions with mass below 1 MeV that decay after the era of CMB last scatter, from their contribution to cold or hot dark matter or $N_{\rm eff}$ at this era.