CP asymmetry in heavy Majorana neutrino decays at finite temperature: the hierarchical case

TL;DR

This paper calculates the CP asymmetry in heavy Majorana neutrino decays at finite temperature within a hierarchical mass scenario, using effective field theories to include thermal corrections relevant for leptogenesis.

Contribution

It introduces a systematic effective field theory approach to compute thermal CP asymmetry in hierarchical Majorana neutrino decays, including leading corrections from Standard Model couplings.

Findings

Thermal corrections are encoded in a single dimension-five operator.

Corrections proportional to the top Yukawa are computed via four dimension-seven operators.

The approach provides a clear hierarchy of effective theories for leptogenesis calculations.

Abstract

We consider the simplest realization of leptogenesis with one heavy Majorana neutrino species much lighter than the other ones. In this scenario, when the temperature of the early universe is smaller than the lightest Majorana neutrino mass, we compute at first order in the Standard Model couplings and, for each coupling, at leading order in the termperature the CP asymmetry in the decays of the lightest neutrino into leptons and anti-leptons. We perform the calculation using a hierarchy of two effective field theories organized as expansions in the inverse of the heavy-neutrino masses. In the ultimate effective field theory, leading thermal corrections proportional to the Higgs self coupling and the gauge couplings are encoded in one single operator of dimension five, whereas corrections proportional to the top Yukawa coupling are encoded in four operators of dimension seven, which we compute.