Flavour Issues in Leptogenesis

TL;DR

This work examines how lepton flavour dynamics alter thermal leptogenesis, incorporating quantum oscillations in flavour space via a density-matrix formalism. By deriving and analyzing flavour-sensitive Boltzmann equations, it shows that oscillations and decoherence from muon Yukawa interactions can significantly affect the evolution of lepton asymmetries, and that individual flavour CP asymmetries can exceed the total. Consequently, the conventional model-independent neutrino-mass bound and the reheat-temperature floor can be relaxed or evaded in many setups, with substantial implications for tying leptogenesis to low-energy neutrino parameters and for compatibility with cosmological constraints such as the gravitino problem. The results indicate that flavour effects are essential for a robust leptogenesis analysis, especially for $oldsymbol{M_1 oughlylesssim 10^{12}}$ GeV, and provide a framework for more complete, flavour-resolved Boltzmann treatments.

Abstract

We study the impact of flavour in thermal leptogenesis, including the quantum oscillations of the asymmetries in lepton flavour space. In the Boltzmann equations we find different numerical factors and additional terms which can affect the results significantly. The upper bound on the CP asymmetry in a specific flavour is weaker than the bound on the sum. This suggests that -- when flavour dynamics is included -- there is no model-independent limit on the light neutrino mass scale,and that the lower bound on the reheat temperature is relaxed by a factor ~ (3 - 10).