Gravitational tests of electroweak relaxation

TL;DR

This paper explores a relaxion-based mechanism for stabilizing the electroweak scale during inflation, analyzing the effects of high reheating temperatures on the back-reaction potential and potential gravitational wave signals from phase transitions.

Contribution

It introduces a scenario with a second relaxation phase triggered by deconfinement, identifying parameter regions that preserve electroweak stability and linking phase transitions to gravitational wave production.

Findings

Second relaxation phase can occur without destabilizing the electroweak scale

High reheating temperatures can temporarily eliminate the back-reaction potential

Strong first order phase transition can produce detectable gravitational waves

Abstract

We consider a scenario in which the electroweak scale is stabilized via the relaxion mechanism during inflation, focussing on the case in which the back-reaction potential is generated by the confinement of new strongly interacting vector-like fermions. If the reheating temperature is sufficiently high to cause the deconfinement of the new strong interactions, the back-reaction barrier then disappears and the Universe undergoes a second relaxation phase. This phase stops when the temperature drops sufficiently for the back-reaction to form again. We identify the regions of parameter space in which the second relaxation phase does not spoil the successful stabilization of the electroweak scale. In addition, the generation of the back-reaction potential that ends the second relaxation phase can be associated to a strong first order phase transition. We then study when such transition can generate a gravitational wave signal in the range of detectability of future interferometer experiments.