The nuMSM, leptonic asymmetries, and properties of singlet fermions
Mikhail Shaposhnikov
TL;DR
This paper analyzes baryogenesis in the νMSM, a minimal SM extension with three light sterile neutrinos, by formulating a density-matrix kinetic framework that captures CP violation, quantum coherence, and damping in the early Universe. It identifies the CP-violating phase that drives baryogenesis, shows that coherence between the nearly degenerate heavy neutrinos is essential for generating a lepton asymmetry, and maps the viable mass and Yukawa-coupling space across several mass-difference scenarios. The results indicate that successful baryogenesis is possible for a broad range of heavy-neutrino masses with near-degeneracy, and that large lepton asymmetries at low temperatures—which can enhance resonant production of dark matter sterile neutrinos—are achievable only in specific parameter regions (notably Scenario IIa with tuned mass splitting). The work connects these cosmological outcomes to potential accelerator and astrophysical tests, and discusses fine-tunings and possible underlying symmetries that could explain the required parameter patterns, suggesting νMSM as a coherent framework for neutrino masses, baryogenesis, and dark matter in a single model.
Abstract
We study in detail the mechanism of baryon and lepton asymmetry generation in the framework of the $ν$MSM (an extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale). We elucidate the issue of CP-violation in the model and define the phase relevant for baryogenesis. We clarify the question of quantum-mechanical coherence, essential for the lepton asymmetry generation in singlet fermion oscillations and compute the relevant damping rates. The range of masses and couplings of singlet leptons which can lead to successful baryogenesis is determined. The conditions which ensure survival of primordial (existing above the electroweak temperatures) asymmetries in different leptonic numbers are analysed. We address the question whether CP-violating reactions with lepton number non-conservation can produce leptonic asymmetry {\em below} the sphaleron freeze-out temperature. This asymmetry, if created, leads to resonant production of dark matter sterile neutrinos. We show that the requirement that a significant lepton asymmetry be produced puts stringent constraints on the properties of a pair of nearly degenerate singlet fermions, which can be tested in accelerator experiments. In this region of parameters the $ν$MSM provides a common mechanism for production of baryonic matter and dark matter in the universe. We analyse different fine-tunings of the model and discuss possible symmetries of the $ν$MSM Lagrangian that can lead to them.