Dark matter from axion and small neutrino mass

Abstract: We explore a KSVZ-like extension of the Standard Model with a Dirac fermion and three right-handed neutrinos. PQ symmetry allows the Dirac mass for neutrinos and prevents the Majorana mass. A $\mathcal{Z}_2$ symmetry guarantees the stability of Dirac fermion dark matter. The breakdown of PQ symmetry generates the QCD axion at a high scale. The fermion dark matter relic abundance arises from the UV-freeze-in mechanism through the axion portal. We determine the fermion DM relic by solving the coupled Boltzmann equations and finding the allowed parameter space using the relic density constraints. Having determined the allowed parameter space for fermion DM, we also look for non-thermal axion production schemes to seek the two DM possibility. We find that FIMP alone is a suitable dark matter that is not excluded while considering several current bounds and future sensitivities on axion and dark matter. Our study highlights the interlinking of dark matter, axion, and neutrinos while addressing the strong CP problem and small neutrino masses.