Axion-Gauge Coupling Quantization with a Twist

TL;DR

This paper explores how the global structure of the gauge group influences axion couplings to gauge fields, revealing quantization conditions that relate photon and gluon couplings, and identifying a specific coupling value of experimental interest.

Contribution

It establishes a correlation between axion-gauge couplings based on gauge group structure, deriving quantization conditions and identifying the minimal axion-photon coupling scenario.

Findings

Photon coupling can have fractional parts in multiples of 1/3.

The smallest axion-photon amplitude occurs at E/N = 8/3.

Quantization conditions depend on the gauge group's global structure.

Abstract

The possible couplings of an axion to gauge fields depend on the global structure of the gauge group. If the Standard Model gauge group is minimal, or equivalently if fractionally charged color-singlet particles are forbidden, then the QCD axion's Chern-Simons couplings to photons and gluons obey correlated quantization conditions. Specifically, the photon coupling can have a fractional part which is a multiple of 1/3, but which is determined by the gluon coupling. A consequence of this result is that, among all theories with a minimal gauge group and minimal axion coupling to gluons, the smallest possible axion-photon amplitude $|g_{a\gamma\gamma}|$ arises for $E/N = 8/3$. This provides a new motivation for experiments targeting this axion-photon coupling.