The Too Visible QCD Axion
arXiv:2602.10057
The paper examines a GeV‑scale QCD axion model where the up‑quark mass arises from the QCD chiral condensate, identifies a structural problem that breaks isospin symmetry and distorts pion scattering, and shows that a light axion is ruled out by fifth‑force experiments.
Abstract
Murayama proposed a GeV-scale axion theory where the up-quark mass term is generated dynamically by the QCD chiral condensate, spontaneously breaking a Peccei-Quinn symmetry. It predicts a too large mass splitting between neutral and charged pions. Trying to solve this problem we explore extensions. Despite some partial improvements, we identify a structural obstruction: the new Peccei-Quinn spurion breaks the accidental isospin symmetry of the chiral Lagrangian, leading to an enhanced higher-order operator. As a consequence, pion scatterings too are distorted. We also examine the limit in which the axion becomes light, finding that it is excluded by fifth-force constraints.
v3: note added about why extending the mechanism to heavy quarks is not viable. Webinar presentation at https://www.youtube.com/watch?v=2mXw1RDbS3U