Electroweak Baryogenesis and Standard Model CP Violation

TL;DR

This work critically assesses electroweak baryogenesis driven solely by the CKM CP-violating phase. It reframes quark scattering off a moving bubble wall in a hot plasma as a problem of quasiparticle coherence, introducing a coherence length {\ell} set by the damping rate {\gamma}. By solving the Dirac equation for quasiparticles and summing over damped flavor-changing paths, the authors show that quantum interference required for CP violation is strongly suppressed, yielding |n_B/s| well below observational values. The result challenges the viability of SM-based baryogenesis and argues that CKM CP violation is insufficient unless new sources of CP violation or beyond-SM physics are invoked.

Abstract

We analyze the mechanism of electroweak baryogenesis proposed by Farrar and Shaposhnikov in which the phase of the CKM mixing matrix is the only source of $CP$ violation. This mechanism is based on a phase separation of baryons via the scattering of quasiparticles by the wall of an expanding bubble produced at the electroweak phase transition. In agreement with the recent work of Gavela, Hernández, Orloff and Pène, we conclude that QCD damping effects reduce the asymmetry produced to a negligible amount. We interpret the damping as quantum decoherence. We compute the asymmetry analytically. Our analysis reflects the observation that only a thin, outer layer of the bubble contributes to the coherent scattering of the quasiparticles. The generality of our arguments rules out any mechanism of electroweak baryogenesis that does not make use of a new source of $CP$ violation.

Figures (1)

• Figure 4: First two terms in the expansion for the reflection matrix $R_{LR}.$ The bubble of broken phase is indicated by the step. An incident left-handed quasiparticle approaches the bubble from the left, and is scattered by the quark-mass term ${\cal M}$ in the bubble, becoming a right-handed quasiparticle which moves back towards the bubble wall. The right-handed particle can then exit the bubble and contribute to the reflected wave, or else can scatter again, via ${\cal M}^{\dagger},$ leading to a contribution to the reflected wave at higher order in the quark mass matrix. The full reflected wave is obtained by summing up these diagrams and integrating over the positions of the scatterings in the bubble.