Non-local Electroweak Baryogenesis Part II : The Classical Regime

Abstract

We investigate baryogenesis at a first order electroweak phase transition in the presence of a CP violating condensate on the bubble walls, in the regime in which the bubble walls are `thick', in the sense that fermions interact with the plasma many times as the bubble wall passes. Such a condensate is present in multi-Higgs extensions of the standard model and may be formed via an instability in the minimal standard model. We concentrate on particles with typical thermal energies in the plasma, whose interactions with the wall are accurately described by the WKB approximation, in which a classical chiral force is evident. The deviations from thermal equilibrium produced by motion of the wall are then treated using a classical Boltzmann equation which we solve in a fluid approximation. From the resulting equations we find two effects important for baryogenesis: (i) a classical chiral force term due to the $CP$ violating background, and (ii) a term arising from hypercharge violating interactions which are pushed out of equilibrium by the background{field. Provided the wall propagates slower than the speed of sound, both terms lead to the diffusion of a chiral asymmetry in front of the wall. This can produce a baryon asymmetry of the observed magnitude for typical wall velocities and thicknesses.