Magnetic anomaly coefficients for QCD axion couplings

TL;DR

This paper revisits KSVZ-like axion models, highlighting the importance of magnetic anomalies in determining axion-photon couplings and proposing a simplified, more general framework that aligns better with astrophysical data.

Contribution

It introduces magnetic charge into KSVZ models and derives magnetic anomaly coefficients, providing a more comprehensive understanding of axion-photon interactions.

Findings

Magnetic anomalies dominate axion-photon coupling.

Allowing magnetic charges simplifies the model.

Derived new expressions for magnetic anomaly coefficients.

Abstract

We argue that there are both experimental and theoretical reasons to reconsider the construction of KSVZ-like axion models. From the experimental side, predictions for the axion-photon coupling are not consistent with the collection of astrophysical hints. From the theoretical side, we find that the construction can be conceptually simplified. In particular, it contains an unnecessary assumption on the type of the gauge theory involved which has far-reaching consequences for low energy axion phenomenology. In order to relax this assumption, we allow heavy fermions of the KSVZ-like models to carry magnetic charge. We then show that the axion-photon coupling is generically dominated by the axial anomaly of magnetic currents. Finally, we derive the expression for magnetic anomaly coefficients, which determine the range of numerical values for the axion-photon coupling.