A bstract We construct explicit models that solve the axion quality problem originating from quantum gravitational effects. The general strategy we employ is to supplement the Standard Model and its grand unified extensions by an anomaly-free axial U(1) a symmetry that is gauged. We show that for several choices of the gauge quantum numbers of the fermions, this setup leads to an accidental U(1) symmetry with a QCD anomaly which can be identified as the Peccei-Quinn (PQ) symmetry that solves the strong CP problem. The U(1) a gauge symmetry controls the amount of explicit PQ symmetry violation induced by quantum gravity, resulting in a high quality axion. We present two classes of models employing this strategy. In the first class (models I and II), the axial U(1) a gauge symmetry acts on vector-like quarks leading to an accidental KSVZ-type axion. The second class (model III) is based on SO(10) grand unified theory extended by a gauged U(1) a symmetry that leads to a hybrid KSVZ-DFSZ type axion. The couplings of the axion to the electron and the nucleon are found to be distinct in this class of hybrid models from those in the KSVZ and DFSZ models, when the axion is identified as the dark matter of the universe, which can be used to test these models. Interestingly, all models presented here have domain wall number of one, which is free of cosmological problems that typically arise in axion models.
Babu et al. (2026) studied this question.