Paper detail

An efficient approach to electroweak bubble velocities

Extensions of the Standard Model are being considered as viable settings for a first-order electroweak phase transition which satisfy Sakharov's three conditions for the generation of the baryon asymmetry of the Universe. These extensions provide a sufficiently strong phase transition and remove the main obstacles which appear in the context of the Standard Model: A far-too-high lower bound on the Higgs mass, immediate wipeout of the newly-created baryon asymmetry, and insufficient CP violation. We describe the Universe hydrodynamically as a fluid coupled to the Higgs field via a phenomenological friction term, and study the time evolution of bubbles nucleated during the phase transition. We express the friction term in the hydrodynamic equations in terms of the particle content of the model, calibrate the friction on the basis of existing calculations for the Standard Model, and produce predictions for the velocity of the expanding bubble wall in the stationary regime. This way we develop a very efficient approach to compute bubble velocities. As an example, we apply our formalism to the first-order phase transition of a dimension-6 extension of the Standard Model which, within the present bounds on the Higgs mass, can reproduce the observed baryon asymmetry of the Universe. Depending on the strength of the phase transition, the wall velocity varies from about 0.3 to approaching the speed of light. Our method can easily be adapted to compute wall velocities in other interesting extensions of the Standard Model.