Dynamics of the Peccei Quinn Scale

Understand

Invoking the Peccei-Quinn (PQ) solution to the strong CP problem substitutes the puzzle of why $\theta_{qcd}$ is so small with the puzzle of why the PQ symmetry is of such high quality.

• Cosmological and astrophysical considerations raise further puzzles.
• This paper explores this issues in several contexts: string theory and field theory, and theories without and with low energy supersymmetry.
• Among the questions studied are whether requiring axion dark matter can account for the quality of the PQ symmetry, to which the answer is sometimes yes.