Tri-Resonant Leptogenesis in a Seesaw Extension of the Standard Model

TL;DR

This paper investigates a leptogenesis model with three nearly degenerate heavy Majorana neutrinos, demonstrating enhanced CP asymmetries and emphasizing the importance of temperature-dependent plasma effects for accurate baryon asymmetry predictions at sub-100 GeV scales.

Contribution

It introduces a tri-resonant leptogenesis framework with detailed Boltzmann equation analysis including plasma effects, extending understanding of low-mass leptogenesis.

Findings

Enhanced CP asymmetries in tri-resonant neutrino systems.

Significant corrections from temperature-dependent degrees of freedom.

Potential for experimental tests in future lepton flavor violation searches.

Abstract

We study a class of leptogenesis models where the light neutrinos acquire their observed small masses by a symmetry-motivated construction. This class of models may naturally include three nearly degenerate heavy Majorana neutrinos that can strongly mix with one another and have mass differences comparable to their decay widths. We find that such a tri-resonant heavy neutrino system can lead to leptonic CP asymmetries which are further enhanced than those obtained in the usual bi-resonant approximation. Moreover, we solve the Boltzmann equations by paying special attention to the temperature dependence of the relativistic degrees of freedom of the plasma. The latter results in significant corrections to the evolution equations for the heavy neutrinos and the lepton asymmetry that have been previously ignored in the literature. We show the importance of these corrections to accurately describe the dynamical evolution of the baryon-to-photon ratio $\eta_B$ for heavy neutrino masses at and below $100$ GeV, and demonstrate that successful leptogenesis at lower masses can be significantly affected by the variation of the relativistic dofs. The parameter space for the leptogenesis model is discussed, and it could be probed in future experimental facilities searching for charged lepton flavour violation and heavy neutrinos in future $Z$-boson factories.