Relaxion Cosmology and the Price of Fine-Tuning

TL;DR

This paper integrates the relaxion mechanism with a novel inflationary cosmology model, demonstrating how inflationary constraints impact the naturalness and fine-tuning of the theory.

Contribution

It proposes a relaxion hybrid inflation model with an asymmetric waterfall, connecting relaxion dynamics to observable inflationary parameters.

Findings

The inflaton sector increases the fine-tuning of the model.

The proposed scenario is consistent with observational constraints.

Inflationary dynamics influence the relaxion's role in solving the hierarchy problem.

Abstract

The relaxion scenario presents an intriguing extension of the standard model in which the particle introduced to solve to the strong CP problem, the axion, also achieves the dynamical relaxation of the Higgs boson mass term. In this work we complete this framework by proposing a scenario of inflationary cosmology that is consistent with all the observational constraints: the relaxion hybrid inflation with an asymmetric waterfall. In our scheme, the vacuum energy of the inflaton drives inflation in a natural way while the relaxion slow-rolls. The constraints on the present inflationary observables are then matched through a subsequent inflationary epoch driven by the inflaton. We quantify the amount of fine-tuning of the proposed inflation scenario, concluding that the inflaton sector severely decreases the naturalness of the theory.