The nuMSM, leptonic asymmetries, and properties of singlet fermions

TL;DR

This paper investigates how the $ u$MSM model can generate baryon and lepton asymmetries, explores the conditions for successful baryogenesis and dark matter production, and constrains the properties of singlet fermions through detailed theoretical analysis.

Contribution

It provides a detailed analysis of CP-violation, quantum coherence, and parameter constraints in the $ u$MSM for baryogenesis and dark matter production, including potential experimental tests.

Findings

Range of singlet fermion masses and couplings for successful baryogenesis identified.

Conditions for survival of primordial asymmetries analyzed.

Constraints on nearly degenerate singlet fermions for leptogenesis and dark matter production derived.

Abstract

We study in detail the mechanism of baryon and lepton asymmetry generation in the framework of the $\nu$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 $\nu$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 $\nu$MSM Lagrangian that can lead to them.