Electroweak baryogenesis at high bubble wall velocities

Abstract

It is widely believed that electroweak baryogenesis should be suppressed in strong phase transitions with fast-moving bubble walls, but this effect has never been quantitatively studied. We rederive fluid equations describing transport of particle asymmetries near the bubble wall without making the small-wall-velocity approximation. We show that the suppression of the baryon asymmetry is a smooth function of the wall speed and that there is no special behavior when crossing the sound speed barrier. Electroweak baryogenesis can thus be efficient also with strong detonations, generically associated with models with observably large gravitational waves. We also make a systematic and critical comparison of our improved transport equations to another one commonly used in the literature, based on the VEV-insertion formalism.

Fluid equations for fast-moving electroweak bubble walls

“…A similar observation was recently made in Ref. [16] in the context of the CP-odd fluid perturbations that are needed to compute the source terms for EWBG, but the analogous study for the CP-even perturbations, relevant to determining the bubble wall properties, has not been done. It requires more work because the perturbation in the local temperature δτ ¼ δT=T (not needed for the EWBG source terms) must now be included in the network.…”

“…As argued in Ref. [16], this behavior is unphysical, since there is no reason for particles not to be able to diffuse in front as long as their v z velocity component is higher than v.…”

“…The latter has long been recognized and recently highlighted in the high-v context in Ref. [16]. While there are strong theoretical motivations for the velocity perturbation, the choice of weighting factors is more arbitrary, and cannot be justified a priori.…”

“…(3) [24], one can see that this implies that the perturbations q must strictly vanish in front of the wall for v > c s . Reference [16] has argued that this kind of behavior is unphysical, since the fluid equations (3) describe particle diffusion, which is a physically distinct process from the propagation of sound waves. There is no reason why diffusion should be suddenly quenched in the vicinity of a supersonic wall, since some fraction of particles in front of the wall can still travel fast enough to get ahead of it.…”

“…In this section we propose a new formalism (NF) for the fluid equations, motivated by the recent paper [16]. In that work, the problem of artificial suppression of diffusion for supersonic walls was overcome, following a longestablished method of dealing with the velocity perturbation u [25][26][27].…”