Catalogues of Cosmologically Self-Consistent Hadronic QCD Axion Models

Abstract: We extend the catalogue of "phenomenologically preferred" hadronic axion models to include heavy fermion representations associated with higher-dimensional decay operators. The latter have recently been shown to self-consistently trigger a period of early matter domination, making the underlying axion models cosmologically viable. After identifying all possible representations up to decay operator dimension $d \leq 9$, we update the hadronic axion band for the axion-photon coupling. The central regions of the axion band are similar to those found previously and approximately independent of the axion decay constant $f_a$, suggesting that they are robust predictions and targets for future axion searches. Moreover, we find that $d = 6$ and $d = 7$ operators can lead to two new viable "model islands" around $f_a \sim 10^{12}$ GeV and $f_a \sim 10^{14}$ GeV, i.e., beyond the standard post-inflationary mass region.