Dynamics of Electroweak Phase Transition In Singlet-Scalar Extension of the Standard Model

An addition to the Standard Model of a real, gauge-singlet scalar field, coupled via a Higgs portal interaction, can reopen the possibility of a strongly first-order electroweak phase transition (EWPT) and successful electroweak baryogenesis (EWBG). If a discrete symmetry that forbids doublet-singlet mixing is present, this model is notoriously difficult to test at the Large Hadron Collider. As a result, it emerged as a useful benchmark for evaluating the capabilities of proposed future colliders to conclusively test EWPT and EWBG. In this paper, we evaluate the bubble nucleation temperature throughout the parameter space of this model where a first-order transition is expected. We find that in large parts of this parameter space, bubbles in fact do not nucleate at any finite temperature, eliminating these models as viable EWBG scenarios. This constraint eliminates most of the region where a "two-step" phase transition is naively predicted, while the "one-step" transition region is largely unaffected. In addition, expanding bubble walls must not reach relativistic speeds during the transition for baryon asymmetry to be generated. We show that this condition further reduces the parameter space with viable EWBG.