Feebly-Interacting Peccei-Quinn Model

Abstract

The QCD axion is widely studied as a dark matter (DM) candidate and as a solution to the strong CP problem of the Standard Model. In conventional field-theoretic models, a much larger mass scale than the electroweak (EW) scale is typically introduced to spontaneously break Peccei-Quinn (PQ) symmetry with a large enough axion decay constant, $f_a$, thereby avoiding constraints from star cooling. In this paper, I propose an alternative approach to achieving the large decay constant: a PQ scalar field with a large wave function renormalization constant, analogous to a feebly coupled gauge theory. Other dimensionless parameters are ${O}(1)$ in the unit of the EW scale for the naturalness. This framework predicts a light PQ Higgs boson with a mass $\sim (\mathrm{EW~scale})^2 / f_a$. Exotic particles associated with the PQ anomaly are expected to have masses around the EW scale. The proposed model alleviates both the PQ quality and EW scale fine-tuning problems and introduces interesting axion-PQ Higgs cosmologies, encompassing: slim axion DM from a fat string network, heavy axion DM from PQ Higgs condensate fragmentation, PQ Higgs DM, and axion-PQ Higgs co-DM scenarios. Potential experimental signatures are explored, including fifth-force tests, DM detections, accelerator searches, and gravitational wave observations by employing lattice simulation. Possible extensions of the scenario are also discussed.