The Too Visible QCD Axion
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.
Cite
@article{arxiv.2602.10057,
title = {The Too Visible QCD Axion},
author = {Luca Di Luzio and Michele Redi and Alessandro Strumia and Andrea Tesi and Arsenii V. Titov},
journal= {arXiv preprint arXiv:2602.10057},
year = {2026}
}
Comments
v2: 19 pages, appendix and references added. Webinar presentation at https://www.youtube.com/watch?v=2mXw1RDbS3U