Relaxion window

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 [Formula: see text]. We also find that a strong correlation between the Hubble parameter at the relaxion stabilization and the scale [Formula: see text] of non-QCD strong dynamics, which generates the non-perturbative relaxion cosine potential. For a higher relaxion mass, a large scale [Formula: see text] becomes available. However, for its lower mass, [Formula: see text] should be small and constructing such a particle physics model is challenging.