The DFSZ axion in the CMB

Understand

We perform for the first time a dedicated analysis of cosmological constraints on DFSZ QCD axion models.

• Such constructions are especially interesting in light of the recent Xenon-1T excess and of hints from stellar cooling.
• In DFSZ models, for $m_a\gtrsim 0.1$ eV, scatterings of pions and muons can produce a sizable cosmic background of thermal axions, that behave similarly to massive neutrinos.
• However, the pion coupling depends on the alignment between the vevs of two Higgs doublets, and can be significantly suppressed or enhanced with respect to the KSVZ scenario.