HOW FAST CAN THE WALL MOVE - A STUDY OF THE ELECTROWEAK PHASE-TRANSITION DYNAMICS

We consider the dynamics of bubble growth in the minimal standard mode l at the electroweak phase transition and determine the shape and the velocity of the phase boundary, or bubble wall. We show that in the se miclassical approximation the friction on the wall arises from the dev iation of massive particle populations from thermal equilibrium. We tr eat these with Boltzmann equations in a fluid approximation. This appr oximation is reasonable for the top quarks and the light species while it, underestimates the friction from the infrared W bosons and Higgs particles. We use the two-loop finite temperature effective potential and find a subsonic bubble wall for the whole range of Higgs boson mas ses 0 < m(H) < 90 GeV. The result is weakly dependent on m(H): the wal l velocity v(w) falls in the range 0.36 < v(w) < 0.44, while the wall thickness is in the range 29 > LT > 23. The wall is thicker than the p hase equilibrium value because out of equilibrium particles exert more friction on the back than on the base of a moving wall. We also consi der the effect of an infrared gauge condensate which may exist in the symmetric phase; modeling it simple mindedly, we find that the wall ma y become supersonic, but not ultrarelativistic.