Speaker
Description
In a recent paper, Murayama proposed a GeV-scale axion scenario in which the up-quark mass arises dynamically from the QCD chiral condensate, which at the same time spontaneously breaks a Peccei–Quinn symmetry. If correct, this striking idea would imply that the QCD axion has effectively already been discovered and is hidden among the known pseudoscalar resonances in the 1–2 GeV region. The proposal, however, faces a serious challenge: it predicts too large a mass splitting between neutral and charged pions. In this talk, I will discuss several attempts to overcome this problem. Despite some partial improvements, we find a structural obstruction: the new Peccei–Quinn spurion breaks the accidental isospin symmetry of the chiral Lagrangian, generating an enhanced higher-order operator. This in turn also leads to sizable distortions in pion-pion scattering. Although a successful resolution appears unlikely, a definitive conclusion would require lattice simulations of the resulting deformed QCD theory.