Electroweak baryogenesis from light fermion sources: a critical study

TL;DR

This paper critically evaluates the viability of electroweak baryogenesis driven by light fermion sources, comparing different theoretical formalisms and clarifying discrepancies to identify scenarios that could produce a sufficient baryon asymmetry.

Contribution

It provides a detailed comparison of the semiclassical (WKB) and VEV insertion approximation formalisms for light fermion sources in EWBG, highlighting the conditions under which they predict viable baryon asymmetry.

Findings

VIA predicts much larger baryon asymmetry than WKB.

The shape of CP-violating source terms causes cancellations in WKB.

$c$-$t$ quark mixing can produce a realistic baryon asymmetry.

Abstract

Electroweak baryogenesis (EWBG) is sourced by nonstandard $CP$-violating interactions of the Higgs boson with fermions, usually taken to be the top quark, enhanced by its large Yukawa coupling. Numerous papers have studied EWBG sourced by lighter fermions, including the tau lepton and off-diagonal quark mass terms. We critically reassess the viability of EWBG in these scenarios, comparing the predictions based on the semiclassical (WKB) formalism for the source term to those from the VEV insertion approximation (VIA), using updated values for the collision terms, and clarifying discrepancies in the definition of the weak sphaleron rate. The VIA systematically predicts a baryon asymmetry that is orders of magnitude larger than the WKB formalism. We trace this to the differing shapes of the $CP$-violating source terms in the two formalisms, showing that the additional spatial derivative in the WKB source term causes large cancellations when it is integrated over the bubble wall profile. An important exception is a source term from $c$-$t$ quark mixing, where the WKB prediction also allows for a realistically large baryon asymmetry. In contrast, the analogous $b$-$s$ mixing source is found to be orders of magnitude too small.