Can electroweak bubble walls run away?

TL;DR

This paper investigates conditions under which electroweak bubble walls can accelerate to ultra-relativistic speeds during phase transitions in certain extensions of the Standard Model, providing criteria to predict runaway behavior.

Contribution

It introduces a simple criterion to determine whether bubble walls in electroweak phase transitions can become ultra-relativistic or reach a terminal velocity.

Findings

Bubble walls can become ultra-relativistic with gamma growing linearly with distance.

A criterion is provided to predict runaway bubble wall behavior.

Analysis applies to models with SU(2) singlet scalar fields.

Abstract

In extensions of the Standard Model with SU(2) singlet scalar fields, there can be regions of parameter space for which the electroweak phase transition is first order already at the mean-field level of analysis. We show that in this case the phase interface (bubble wall) can become ultra-relativistic, with the relativistic gamma factor gamma = (1-v_{wall}^2)^{-1/2} growing linearly with the wall's propagation distance. We provide a simple criterion for determining whether the bubble wall "runs away" in this way or if gamma approaches a terminal value.