Gravitational waves from conformal symmetry breaking

TL;DR

This paper studies the electroweak phase transition in a conformal extension of the standard model, estimating the resulting gravitational wave spectrum and its detectability by LISA, while addressing theoretical uncertainties.

Contribution

It provides the first estimate of gravitational waves from a conformal electroweak phase transition including supercooling effects and introduces a thermal gap equation approach for better perturbativity.

Findings

Gravitational wave signals are within LISA's observational window.

The phase transition is very strong with significant supercooling.

A new method improves perturbative estimates during the transition.

Abstract

We consider the electroweak phase transition in the conformal extension of the standard model known as SU(2)cSM. Apart from the standard model particles, this model contains an additional scalar and gauge field that are both charged under the hidden SU(2)$_X$. This model generically exhibits a very strong phase transition that proceeds after a large amount of supercooling. We estimate the gravitational wave spectrum produced in this model and show that its amplitude and frequency fall within the observational window of LISA. We also discuss potential pitfalls and relevant points of improvement required to attain reliable estimates of the gravitational wave production in this - as well as in more general - class of models. In order to improve perturbativity during the early stages of transition that ends with bubble nucleation, we solve a thermal gap equation in the scalar sector inspired by the 2PI effective action formalism.