A New Target for Cosmic Axion Searches

TL;DR

Future CMB experiments could significantly advance axion searches by detecting or constraining their couplings to Standard Model particles, surpassing current laboratory and astrophysical bounds.

Contribution

This paper demonstrates the potential of future cosmological observations to probe axion couplings with Standard Model particles at unprecedented sensitivity levels.

Findings

Cosmology can detect light relics at high decoupling temperatures.

Non-detection constrains axion interactions to be very weak.

Cosmological bounds can outperform laboratory and astrophysical limits.

Abstract

Future CMB experiments have the potential to probe the density of relativistic species at the sub-percent level. Sensitivity at this level allows light thermal relics to be detected up to arbitrarily high decoupling temperatures. Conversely, the absence of a detection would require extra light species never to have been in equilibrium with the Standard Model. In this paper, we exploit this feature to demonstrate the sensitivity of future cosmological observations to the couplings of axions to all of the Standard Model degrees of freedom. In many cases, the constraints achievable from cosmology will surpass existing bounds from laboratory experiments and astrophysical observations by orders of magnitude.