Novel cosmological bounds on thermally-produced axion-like particles

Understand

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.