Phenomenology of relaxion-Higgs mixing

TL;DR

This paper explores how relaxion-Higgs mixing occurs at CP-breaking points, leading to diverse experimental signatures across a broad relaxion mass range, and discusses constraints and fine-tuning issues in related models.

Contribution

It demonstrates that relaxion-Higgs mixing naturally occurs at CP-breaking points and analyzes experimental constraints and fine-tuning in multiaxion UV completions.

Findings

Relaxion mass can range from sub-eV to GeV scale.

Current experiments already constrain significant parameter space.

Multiaxion models face mild fine-tuning issues increasing with the number of sites.

Abstract

We show that the relaxion generically stops its rolling at a point that breaks CP leading to relaxion-Higgs mixing. This opens the door to a variety of observational probes since the possible relaxion mass spans a broad range from sub-eV to the GeV scale. We derive constraints from current experiments (fifth force, astrophysical and cosmological probes, beam dump, flavour, LEP and LHC) and present projections from future experiments such as NA62, SHiP and PIXIE. We find that a large region of the parameter space is already under the experimental scrutiny. All the experimental constraints we derive are equally applicable for general Higgs portal models. In addition, we show that simple multiaxion (clockwork) UV completions suffer from a mild fine tuning problem, which increases with the number of sites. These results favour a cut-off scale lower than the existing theoretical bounds.