Anomaly Ratio Distributions of Hadronic Axion Models with Multiple Heavy Quarks

TL;DR

This paper classifies and analyzes the distributions of anomaly ratios in hadronic axion models with multiple heavy quarks, providing insights into model preferences and implications for axion-photon coupling constraints.

Contribution

It systematically catalogs all possible models with up to 28 heavy quarks, analyzes their anomaly ratio distributions, and discusses implications for axion phenomenology.

Findings

Only 820 distinct anomaly ratios $E/N$ exist for models with up to 28 heavy quarks.

The $E/N$ distributions can inform priors on axion-photon coupling.

The model band aligns with current and future experimental constraints.

Abstract

We consider hadronic axion models that extend the Standard Model by one complex scalar field and one or more new heavy quarks, i.e. $N_\mathcal{Q} \geq 1$. We review previously suggested selection criteria as well as categorize and catalog all possible models for $N_\mathcal{Q} \leq 9$. In particular, allowing for $N_\mathcal{Q} > 1$ can introduce models that spoil the axion solution of the strong CP problem. Demanding that Landau poles do not appear below some energy scale limits the number of preferred models to a finite number. For our choice of criteria, we find that $N_\mathcal{Q} \leq 28$ and only 820 different anomaly ratios $E/N$ exist (443 when considering additive representations, 12 when all new quarks transform under the same representation). We analyze the ensuing $E/N$ distributions, which can be used to construct informative priors on the axion-photon coupling. The hadronic axion model band may be defined as the central region of one of these distributions, and we show how the band for equally probable, preferred models compares to present and future experimental constraints.