Wall speed and shape in singlet-assisted strong electroweak 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.