Novel cosmological bounds on thermally-produced axion-like particles

TL;DR

This paper derives new cosmological bounds on thermally-produced axion-like particles by analyzing CMB and BAO data, improving existing limits on their couplings and masses, and discussing future observational prospects.

Contribution

It provides the most stringent cosmological bounds to date on axion-gluon coupling and competitive limits on axion-photon coupling, considering recent advances and future CMB experiments.

Findings

New bounds on axion-gluon coupling for 10^{-4} to 100 eV mass range.

Competitive bounds on axion-photon coupling for masses above 3 eV.

Forecasts for future CMB-S4 sensitivity to axion parameters.

Abstract

We constrain the coupling of thermally-produced axion-like particles (here axions) with photons and gluons, using data from the cosmic microwave background (CMB) spectra and baryon acoustic oscillations. The axion possesses an explicit soft breaking mass term and it is produced thermally in the early Universe from either axion-photon or axion-gluon processes, accounting for the recent progresses in the field. We derive the most stringent bounds on the axion-gluon coupling to date on the mass range considered $10^{-4} \lesssim m_a/{\rm eV} \lesssim 100$, superseding the current bounds from SN1987A. The bounds on the axion-photon coupling are competitive with the results from the CAST collaboration for the axion mass $m_a \gtrsim 3\,$eV. We comment on the forecast reaches that will be available given the sensitivity of future CMB-S4 missions.