Redefining the Axion Window

TL;DR

This paper refines the axion parameter space by classifying heavy quark representations in hadronic axion models, establishing a more precise phenomenological window for axion-photon coupling based on theoretical and observational constraints.

Contribution

It introduces a classification of heavy quark representations that define a more precise axion window, considering phenomenological constraints like quark lifetime and absence of Landau poles.

Findings

Identifies 15 heavy quark representations satisfying key constraints.

Defines a new axion window with coupling bounds twice and four times larger than previous estimates.

Shows that multiple representations and decoupling scenarios expand the viable axion parameter space.

Abstract

A major goal of axion searches is to reach inside the parameter space region of realistic axion models. Currently, the boundaries of this region depend on somewhat arbitrary criteria, and it would be desirable to specify them in terms of precise phenomenological requirements. We consider hadronic axion models and classify the representations $R_Q$ of the new heavy quarks $Q$. By requiring that $i)$ the $Q$ are sufficiently short lived to avoid issues with long lived strongly interacting relics, $ii)$ no Landau poles are induced below the Planck scale, fifteen cases are selected, which define a phenomenologically preferred axion window bounded by a maximum (minimum) value of the axion-photon coupling about twice (four times) larger than commonly assumed. Allowing for more than one $R_Q$, larger couplings, as well as complete axion-photon decoupling, become possible.