Systematic moment expansion for electroweak baryogenesis

TL;DR

This paper introduces a systematic moment expansion method for solving semiclassical Boltzmann equations in electroweak baryogenesis, improving accuracy in calculating the baryon asymmetry of the universe.

Contribution

It develops a new moment expansion approach in powers of adiabatic velocity, with a novel truncation scheme that enhances numerical robustness for baryogenesis calculations.

Findings

Higher moment calculations can significantly alter the predicted BAU.

The proposed truncation scheme improves numerical stability over previous methods.

Application to a benchmark model demonstrates the method's effectiveness.

Abstract

We present a systematic moment expansion for solving the semiclassical Boltzmann equations for electroweak baryogenesis. The expansion is developed in powers of adiabatic coordinate velocity, and it is used for computing the CP-violating seed asymmetry at the front of the phase transition wall, that sources the eventual baryon asymmetry of the universe (BAU). We implement the method in a benchmark model, with a CP-violating mass arising from a dimension-5 operator coupling the fermion to a singlet scalar field. We find that the higher moment calculations yield a BAU that can significantly differ from the commonly used two-moment approximation. We discuss in detail the underlying approximations in the moment method and propose a new truncation scheme for the expansion, that appears to give more numerically robust results than the previous schemes.