BUBBLE WALL VELOCITY IN A FIRST-ORDER ELECTROWEAK PHASE-TRANSITION

We calculate the velocity and thickness of a bubble wall at the electr oweak phase transition in the minimal standard model. We model the wal l with semiclassical equations of motion and show that friction arises from the deviation of massive particle populations from thermal equil ibrium. We treat these with Boltzmann equations in a fluid approximati on. Our technique allows us to account for particle transport, and to determine the wall thickness dynamically. We find that the wall is sig nificantly thicker than at phase equilibrium, and that the velocity is fairly high, upsilon(W) similar or equal to 0.6c for m(H) = 50 GeV.