Robustness of accidental Peccei–Quinn symmetry mechanisms in existing composite axion models

Determine the robustness of the mechanisms proposed in existing composite QCD axion models that generate an accidental Peccei–Quinn symmetry via strong dynamics, given that the underlying confining dynamics in many of these models is not well understood. The goal is to assess whether these mechanisms reliably protect the Peccei–Quinn symmetry from explicit breaking to the level required to solve the strong CP problem.

Background

Composite axion models aim to solve the axion quality problem by realizing Peccei–Quinn symmetry as an accidental symmetry of an underlying strongly coupled sector. Several proposals in the literature rely on chiral or confining dynamics to generate and protect the symmetry.

The authors note that for many such models, the detailed dynamics of the strong sector is not well understood, raising concerns about whether the accidental symmetry is genuinely robust against explicit breaking effects (e.g., Planck-suppressed operators). Their paper provides an alternative QCD-like construction designed to ensure high axion quality, but the reliability of prior mechanisms remains an unresolved question in the broader field.