Higgs relaxation after inflation

TL;DR

This paper demonstrates that the cosmological relaxation of the electroweak scale can occur independently of inflation, using gauge boson production to stop the relaxion, allowing higher cutoff scales and subplanckian field excursions.

Contribution

It introduces a relaxion mechanism that operates after inflation, utilizing gauge boson production for stopping, and achieves higher cutoff scales with subplanckian relaxion field excursions.

Findings

Relaxion mechanism can work post-inflation.

Cutoff scale up to 200 TeV is achievable.

Relaxion mass ranges from 200 MeV to the weak scale.

Abstract

We show that the mechanism of cosmological relaxation of the electroweak scale can take place independently of the inflation mechanism, thus relieving burdens from the original relaxion proposal. What eventually stops the (fast-rolling) relaxion field during its cosmological evolution is the production of particles whose mass is controlled by the Higgs vacuum expectation value. We first show that Higgs particle production does not work for that purpose as the Higgs field does not track the minimum of its potential in the regime where Higgs particles get efficiently produced through their coupling to the relaxion. We then focus on gauge boson production. We provide a detailed analysis of the scanning and stopping mechanism and determine the parameter space for which the relaxion mechanism can take place after inflation, while being compatible with cosmological constraints, such as the relaxion dark matter overabundance and Big Bang Nucleosynthesis. We find that the cutoff scale can be as high as two hundreds of TeV. In this approach, the relaxion sector is responsible for reheating the visible sector. The stopping barriers of the periodic potential are large and Higgs-independent, facilitating model-building. The allowed relaxion mass ranges from 200 MeV up to the weak scale. In this scenario, the relaxion field excursion is subplanckian, and is thus many orders of magnitude smaller than in the original relaxion proposal.