Quality of the Peccei-Quinn symmetry in the Aligned QCD Axion and Cosmological Implications

TL;DR

This paper demonstrates that aligned QCD axion models can naturally maintain a high-quality Peccei-Quinn symmetry, with implications for axion cosmology, potential collider signals, and explaining recent experimental anomalies.

Contribution

It introduces a mechanism where multiple axions with small decay constants preserve the PQ symmetry quality despite Planck-scale breaking, affecting cosmology and collider phenomenology.

Findings

Effective strong CP phase remains small despite symmetry breaking.

Domain wall dynamics suppress axion isocurvature perturbations.

Potential collider signals include axions and saxions, possibly explaining the 750 GeV diphoton excess.

Abstract

We show that the required high quality of the Peccei-Quinn symmetry can be naturally explained in the aligned QCD axion models where the QCD axion arises from multiple axions with decay constants much smaller than the axion window, e.g., around the weak scale. Even in the presence of general Planck-suppressed Peccei-Quinn symmetry breaking operators, the effective strong CP phase remains sufficiently small in contrast to the standard axion models without the alignment. The QCD axion potential has small or large modulations due to the symmetry breaking operators, which can significantly affect the axion cosmology. When the axions are trapped in different minima, domain walls appear and their scaling behavior suppresses the axion isocurvature perturbations at super-horizon scales. Our scenario predicts many axions and saxions coupled to gluons, and they may be searched for at collider experiments. In particular, the recently found diphoton excess at 750 GeV could be due to one of such (s)axions.