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 also spontaneously breaks a Peccei-Quinn symmetry. If correct, the QCD axion may already be hidden among the known pseudoscalar resonances in the 1-2 GeV region, accompanied by a scalar partner predicted by the same framework. The proposal, however, faces a serious challenge: it generically predicts too large a neutral-charged pion mass splitting. I will discuss attempts to overcome this problem and show that a structural obstruction remains: the new Peccei-Quinn spurion breaks the accidental isospin symmetry of the chiral Lagrangian, generating enhanced higher-order effects and sizable distortions in pion-pion scattering. Although a successful resolution appears unlikely, a definitive assessment of the scenario would require lattice simulations of the resulting deformed QCD theory.