DFSZ Axion Couplings Revisited

TL;DR

This paper revisits the calculations of DFSZ axion couplings, clarifying the methods for obtaining photon and fermion couplings, especially in extended models with multiple Higgs doublets, and discusses flavor conservation and violation.

Contribution

It demonstrates the equivalence of different calculation methods for axion-photon coupling and provides a detailed analysis of axion-fermion couplings in extended DFSZ models.

Findings

Different methods yield consistent axion-photon coupling results.

Axion-fermion couplings depend on vacuum expectation values, expanding parameter space.

Flavor conserving couplings are maintained in multi-Higgs models, with possible flavor violation.

Abstract

Among many possibilities, solar axion has been proposed to explain the electronic recoil events excess observed by Xenon1T collaboration, although it has tension with astrophysical observations. The axion couplings, to photon $g_{a\gamma}$ and to electron $g_{ae}$ play important roles. These couplings are related to the Peccei-Quinn (PQ) charges $X_f$ for fermions. In most of the calculations, $g_{a\gamma}$ is obtained by normalizing to the ratio of electromagnetic anomaly factor $E = TrX_f Q^2_f N_c$ ($N_c$ is 3 and 1 for quarks and charged leptons respectively) and QCD anomaly factor $N = TrX_q T(q)$ ($T(q)$ is quarks' $SU(3)_c$ index). The broken PQ symmetry generator is used in the calculation which does not seem to extract out the components of broken generator in the axion which are "eaten" by the $Z$ boson. However, using the physical components of axion or the ratio of anomaly factors should obtain the same results in the DFSZ for $g_{a\gamma}$. When going beyond the standard DFSZ models, such as variant DFSZ models, where more Higgs doublets and fermions have different PQ charges, one may wonder if the results are different. We show that the two methods obtain the same results as expected, but the axion couplings to quarks and leptons $g_{af}$ (here f indicates one of the fermions in the SM) are more conveniently calculated in the physical axion basis. The result depends on the values of the vacuum expectation values leading to a wider parameter space for $g_{af}$ in beyond the standard DFSZ axion. We also show explicitly how flavor conserving $g_{af}$ couplings can be maintained when there are more than one Higgs doublets couple to the up and down fermion sectors in variant DFSZ models at tree level, and how flavor violating couplings can arise.