The axion solution to the strong CP problem is only as robust as the Peccei-Quinn symmetry it relies on, and quantum gravity is expected to break it. We quantify the resulting axion quality problem using principled measures of fine-tuning, and show that the cost is severe. For the QCD axion to account for the observed dark matter, Planck-suppressed symmetry-breaking operators must be absent up to mass dimension $d\\ge 12$; the naturalness penalty for failing to realize this protection can exceed a Bayes factor of $10^{10}$. Extending the analysis to generic axion-like particles, we map the axion mass-decay constant plane by the degree of UV protection required, and find that large portions of the sensitivity reach of laboratory experiments are already fine-tuned at the part-per-million level or worse. We argue that quality, not mass, is the central naturalness question for the axion program.
Period
18 May 2026
Event title
2026 Workshop on New Physics and Interdisciplinary Sciences (NPhiS 2026)