Experimental Targets for Photon Couplings of the QCD Axion

TL;DR

This paper explores various mechanisms that can significantly extend the range of photon couplings of the QCD axion, challenging the traditional narrow band assumption and broadening experimental search prospects.

Contribution

It introduces new models, including generalized alignment and clockwork mechanisms, and shows how kinetic mixing can alter axion-photon couplings, expanding the theoretical landscape.

Findings

Multiple mechanisms extend the allowed axion-photon coupling range.

Models fully cover the parameter space accessible to experiments.

Kinetic mixing can produce non-quantized couplings, affecting detection strategies.

Abstract

The QCD axion's coupling to photons is often assumed to lie in a narrow band as a function of the axion mass. We demonstrate that several simple mechanisms, in addition to the photophilic clockwork axion already in the literature, can significantly extend the allowed range of couplings. Some mechanisms we present generalize the KNP alignment scenario, widely studied as a model of inflation, to the phenomenology of a QCD axion. In particular we present KSVZ-like realizations of two-axion KNP alignment and of the clockwork mechanism. Such a "confinement tower" realization of clockwork may prove useful in a variety of model-building contexts. We also show that kinetic mixing of the QCD axion with a lighter axion-like particle can dramatically alter the QCD axion's coupling to photons, differing from the other models we present by allowing non-quantized couplings. The simple models that we present fully cover the range of axion-photon couplings that could be probed by experiments. They motivate growing axion detection efforts over a wide space of masses and couplings.