Wall speed and shape in singlet-assisted strong electroweak phase transitions

TL;DR

This paper develops a new algorithm to accurately determine the speed and shape of bubble walls in singlet-extended models of the electroweak phase transition, with implications for baryogenesis and gravitational waves.

Contribution

It introduces a self-consistent method to compute bubble wall properties in Z2-symmetric singlet models, improving upon previous approaches.

Findings

Wall speeds as low as 0.1 were found in the parameter space.

Subsonic walls require the singlet to be relatively light, m_s  135 GeV.

The algorithm effectively explores the singlet parameter space for strong phase transitions.

Abstract

Models with singlet fields coupling to the Higgs can enable a strongly first order electroweak phase transition, of interest for baryogenesis and gravity waves. We improve on previous attempts to self-consistently solve for the bubble wall properties -- wall speed $v_w$ and shape -- in a highly predictive class of models with $Z_2$-symmetric singlet potentials. A new algorithm is implemented to determine $v_w$ and the wall profiles throughout the singlet parameter space in the case of subsonic walls, focusing on models with strong enough phase transitions to satisfy the sphaleron washout constraint for electroweak baryogenesis. We find speeds as low as $v_w \cong 0.1$ in our scan over parameter space, and the singlet must be relatively light to have a subsonic wall, $m_s \lesssim 135$ GeV.