Freeze-out of baryon number in low-scale leptogenesis

TL;DR

This paper investigates the validity of the standard approximation used in low-scale leptogenesis for baryon asymmetry calculations, revealing significant corrections in some parameter regimes and proposing an improved method.

Contribution

It provides a detailed analysis of the sphaleron freeze-out process in low-scale leptogenesis and introduces a refined approach to account for non-instantaneous effects.

Findings

Corrections to the standard approach can be sizeable for certain parameters.

A feasible improvement to the usual procedure is proposed.

Full kinetic equations reveal the dynamics of baryon and lepton asymmetries.

Abstract

Low-scale leptogenesis provides an economic and testable description of the origin of the baryon asymmetry of the Universe. In this scenario, the baryon asymmetry of the Universe is reprocessed from the lepton asymmetry by electroweak sphaleron processes. Provided that sphalerons are fast enough to maintain equilibrium, the values of the baryon and lepton asymmetries are related to each other. Usually, this relation is used to find the value of the baryon asymmetry at the time of the sphaleron freeze-out. To put in other words, the formula which is valid only when the sphalerons are fast, is applied at the moment when they are actually switched off. In this paper, we examine the validity of such a treatment. To this end, we solve the full system of kinetic equations for low-scale leptogenesis. This system includes equations describing the production of the lepton asymmetry in oscillations of right-handed neutrinos, as well as a separate kinetic equation for the baryon asymmetry. We show that for some values of the model parameters, the corrections to the standard approach are sizeable. We also present a feasible improvement to the ordinary procedure, which accounts for these corrections.