Axion Couplings in Heterotic String Theory

TL;DR

This paper investigates axion couplings in heterotic string theory, deriving bounds on axion-photon interactions that impact experimental searches and constrain theoretical models.

Contribution

It provides a derivation of axion-gauge boson couplings in heterotic string theory and establishes bounds on axion-photon coupling ratios relevant for experiments.

Findings

The axion-photon coupling to axion mass ratio is bounded by the QCD line in certain heterotic models.

Experimental detection of axions above this bound would challenge many heterotic string models.

The work links anomaly matching in string theory to observable axion properties.

Abstract

We study the coupling of axions to gauge bosons in heterotic string theory. The axion-gauge boson couplings in the low energy 4d theory are derived by matching mixed anomalies between higher-form global symmetries and the zero-form gauge symmetry in the 10d theory. When the standard model gauge group is embedded in a single simple group in the 10d theory -- as is the case for almost all heterotic models studied in the literature -- the ratio of the axion-photon coupling to the axion mass is bounded above by the QCD line. This bound is relevant for a large number of axion searches which have sensitivity to axion parameter space above this line. The discovery of an axion in these searches will rule out a large class of heterotic models, making such a signal challenging to explain within heterotic string theory.