Thermal production of astrophobic axions.

Hot axions are produced in the early Universe via their interactions with Standard Model particles, contributing to dark radiation commonly parameterized as ∆Neff. In standard QCD axion benchmark models, this contribution to ∆Neff is negligible after taking into account astrophysical limits such as the SN1987A bound. We therefore compute the axion contribution to ∆Neff in so-called astrophobic axion models characterized by strongly suppressed axion couplings to nucleons and electrons, in which astrophysical constraints are relaxed and ∆Neff may be sizable. We also construct new astrophobic models in which axion couplings to photons and/or muons are suppressed as well, allowing for axion masses as large as few eV. Most astrophobic models are within the reach of CMB-S4, while some allow for ∆Neff as large as the current upper bound from Planck and thus will be probed by the Simons Observatory. The majority of astrophobic axion models predicting large ∆Neff is also within the reach of IAXO or even BabyIAXO. [ABSTRACT FROM AUTHOR]