Isospin-breaking contribution to the model-independent axion-photon-photon coupling in $U(3)$ chiral theory

TL;DR

This paper calculates the isospin-breaking effects on the model-independent axion-photon-photon coupling within $U(3)$ chiral perturbation theory, revealing a correction exceeding 15% compared to the isospin-symmetric case.

Contribution

It provides a next-to-leading order calculation of the isospin-breaking contribution to the axion-photon-photon coupling, including a complete analysis of meson-axion mixing effects.

Findings

Isospin-breaking correction exceeds 15% of the coupling.

Revised $ ext{π}^0$-$ ext{η}$-$ ext{η'}$-axion mixing analysis.

Enhanced accuracy in axion-photon coupling predictions.

Abstract

We pursue the calculation of the model-independent component of the axion-photon-photon coupling in the $U(3)$ chiral perturbation theory up to next-to-leading order, with the emphasis on the isospin breaking effect. The mixing of the $\pi^0$-$\eta$-$\eta'$-axion system is revised as well by working out the complete linear isospin-breaking terms. Our calculation shows that the isospin-breaking correction to the axion-photon-photon coupling amounts to more than 15%, comparing with the result in the isospin limit.