Constraints on Relaxion Windows

TL;DR

This paper explores the relaxion model's low energy phenomenology, identifying the viable parameter space constrained by cosmological and astrophysical data, and highlights regions testable by upcoming experiments.

Contribution

It defines the cosmological relaxion window considering inflationary constraints and assesses observational bounds, pinpointing parameter regions accessible to future experimental probes.

Findings

Most of the relaxion parameter space with high mass or low decay constant is excluded by current constraints.

A viable parameter region exists with relaxion mass 0.2-10 GeV and decay constant 200 TeV, testable by future experiments.

Future beam dump and EDM experiments can probe significant parts of the remaining relaxion parameter space.

Abstract

We examine the low energy phenomenology of the relaxion solution to the weak scale hierarchy problem. Assuming that the Hubble friction is responsible for a dissipation of the relaxion energy, we identify the cosmological relaxion window which corresponds to the parameter region compatible with a given value of the acceptable number of inflationary $e$-foldings. We then discuss a variety of observational constraints on the relaxion window, including those from astrophysical and cosmological considerations. We find that majority of the parameter space with a relaxion mass $m_\phi\gtrsim 100$ eV or a relaxion decay constant $f\lesssim 10^7$ GeV is excluded by existing constraints. There is an interesting parameter region with $m_\phi\sim \,0.2-10$ GeV and $f\sim\, {\rm few}-200$ TeV, which is allowed by existing constraints, but can be probed soon by future beam dump experiments such as the SHiP experiment, or by improved EDM experiments.