CP violation in scatterings, three body processes and the Boltzmann equations for leptogenesis

TL;DR

This paper derives Boltzmann equations for leptogenesis including decay and scattering processes, explicitly computes CP violating asymmetries, and analyzes their impact on lepton asymmetry under various washout conditions.

Contribution

It introduces a comprehensive framework for including CP violation in scatterings within leptogenesis models, with explicit calculations and analysis of their effects.

Findings

CP asymmetries in scatterings can cancel early and late lepton asymmetries.

Weak washouts allow CP scattering effects to reduce final asymmetry.

Strong washouts diminish the impact of CP scatterings on the final lepton asymmetry.

Abstract

We obtain the Boltzmann equations for leptogenesis including decay and scattering processes with two and three body initial or final states. We present an explicit computation of the CP violating scattering asymmetries. We analyze their possible impact in leptogenesis, and we discuss the validity of their approximate expressions in terms of the decay asymmetry. In scenarios in which the initial heavy neutrino density vanishes, the inclusion of CP asymmetries in scatterings can enforce a cancellation between the lepton asymmetry generated at early times and the asymmetry produced at later times. We argue that a sizeable amount of washout is crucial for spoiling this cancellation, and we show that in the regimes in which the washouts are particularly weak, the inclusion of CP violation in scatterings yields a reduction in the final value of the lepton asymmetry. In the strong washout regimes the inclusion of CP violation in scatterings still leads to a significant enhancement of the lepton asymmetry at high temperatures; however, due to the independence from the early conditions that is characteristic of these regimes, the final value of the lepton asymmetry remains approximately unchanged.