Relaxion window

TL;DR

This paper analyzes cosmological constraints on the relaxion model, determining viable mass and coupling ranges, and exploring the relationship between the Hubble parameter and the strong dynamics scale.

Contribution

It provides the first detailed cosmological analysis of the original relaxion scenario, clarifying parameter ranges and the correlation with the strong dynamics scale.

Findings

Relaxion mass range: 10^{-5} eV to 10^4 eV.

Strong correlation between Hubble parameter and strong dynamics scale.

Higher relaxion mass allows larger strong dynamics scale.

Abstract

We investigate cosmological constraints on the original relaxion scenario proposed by Graham, Kaplan and Rajendran. We first discuss the appropriate sign choice of the terms in the scalar potential, when the QCD axion is the relaxion with a relaxion-inflaton coupling proposed in the original paper. We next derive the cosmologically consistent ranges of the mass and a coupling of the relaxion for both the QCD relaxion and non-QCD relaxion. The mass range is obtained by $10^{-5}$ eV $\ll m_{\phi} \lesssim 10^4$ eV. We also find that a strong correlation between the Hubble parameter at the relaxion stabilization and the scale $\Lambda$ of non-QCD strong dynamics, which generates the non-perturbative relaxion cosine potential. For a higher relaxion mass, a large scale $\Lambda$ becomes available. However, for its lower mass, $\Lambda$ should be small and constructing such a particle physics model is challenging.