Phenomenology of the companion-axion model: photon couplings

TL;DR

This paper explores a model with two coupled QCD axions, analyzing their combined interactions and predicting that future experiments could detect both axions simultaneously, offering new insights into the strong-CP problem.

Contribution

It introduces the phenomenology of a two-axion model, showing how it can be tested with current and future axion search experiments, and predicts the potential discovery of two QCD axions.

Findings

Future axion searches may detect two QCD axions.

Recasting bounds onto the companion-axion parameter space.

The second axion helps address gravitational instanton effects.

Abstract

We study the phenomenology of the 'companion-axion model' consisting of two coupled QCD axions. The second axion is required to rescue the Peccei-Quinn solution to the strong-CP problem from the effects of colored gravitational instantons. We investigate here the combined phenomenology of axion-axion and axion-photon interactions, recasting present and future single-axion bounds onto the companion-axion parameter space. Most remarkably, we predict that future axion searches with haloscopes and helioscopes may well discover two QCD axions, perhaps even within the same experiment.