New axion contribution to the two-photon decays of neutral pions

TL;DR

This paper calculates a new axion-induced correction to neutral pion two-photon decays within chiral perturbation theory, showing it could impact interpretations of experimental data and constrain axion models.

Contribution

It introduces a previously missing one-loop correction involving axion-pion mixing to the pion decay process within SU(2) chiral perturbation theory.

Findings

The correction is proportional to axion-photon coupling and axion mass squared.

For classical axion parameters, the correction is highly suppressed.

The result constrains the QCD axion as an explanation for decay width discrepancies.

Abstract

The presence of axions introduces new diagrams at one-loop order to the two-photon decays of the neutral pion through axion-pion mixing. In this work, we calculate this correction, missing in all current calculations, in the framework of SU(2) chiral perturbation theory. We show that the correction is proportional to the axion-photon coupling and the square of axion mass, which in turn is strongly suppressed by the axion decay constant for the classical space window but may not be negligible for the QCD axion in the MeV or even larger mass range. On the other hand, in combination with the experimental measurement of the decay width of $\pi^0\rightarrow\gamma\gamma$ process, this result rules out the standard QCD axion as an explanation for the possible discrepancy between the chiral perturbation theory prediction and the experimental data.