The nuMSM, leptonic asymmetries, and properties of singlet fermions (0804.4542v2)

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.

• The paper establishes that CP-violating oscillations of nearly degenerate singlet fermions are crucial for generating the observed baryon and lepton asymmetries.
• It employs detailed loop diagram calculations to derive production and annihilation rates, highlighting how temperature-induced coherence loss halts asymmetry generation.
• It constrains the parameter space by showing that the lightest sterile neutrino, with mass >0.3 keV and minimal coupling, can serve as a viable dark matter candidate.

Analysis of Lepton and Baryon Asymmetry Within the $\nu$MSM Framework

The paper of the $\nu$MSM, an extension of the Standard Model incorporating three singlet fermions, provides a compelling framework to address both cosmological and phenomenological challenges. This paper explores the intricacies of lepton and baryon asymmetry generation through oscillations of these singlet fermions, underlining the robustness of CP-violating mechanisms and their associated parameters.

Clarifying the CP-violation in the $\nu$MSM is central to understanding baryogenesis. The research delineates the CP-violating phases pivotal for baryon asymmetry and analyses how these singlet neutrinos' masses and couplings facilitate successful baryogenesis. Employing oscillations of these neutrinos plays a critical role, leveraging significant CP-asymmetries during the oscillation phases, which synchronize with the loss of quantum coherence at certain temperature thresholds.

The $\nu$MSM proposes that the lightest sterile neutrino serves as a dark matter candidate, requiring its mass to be above $0.3$ keV and its coupling to be minimal with other model components. This is bounded by both cosmological data favoring dark matter production and astrophysical X-ray observations. The $N_2$ and $N_3$ neutrinos, on the contrary, necessitate a nearly degenerate mass state to ensure coherent CP-violating oscillations, leading to baryon asymmetry relevant to the current mass constraints derived from accelerator experiments and Big Bang Nucleosynthesis (BBN).

The analysis employs a comprehensive understanding of CP-even and CP-odd perturbations in early universe dynamics, incorporating production and annihilation rates of singlet neutrinos, which are calculated with contributions from complex loop diagrams. These rates highlight the importance of maintaining coherent oscillations for effective lepton asymmetry production. The findings articulate how coherence loss equates to a cessation in asymmetry generation, anchoring the dependency of successful baryogenesis on the temperature-induced synchronization of neutrino oscillations.

Impressively, the work also explores the $\nu$MSM parameter space solution for introducing large lepton asymmetries well below the electroweak scale—a novel requirement to foster the resonant production of dark matter sterile neutrinos. This scenario necessitates stringent conditions on the phase-space of nearly degenerate singlet fermions, an area that experimental advances might soon explore, providing opportunities to test these pivotal predictions.

In conclusion, the research undertaken herein meticulously maps how imposing constraints on singlet neutrino properties—mass, coupling, and CP-violating phases—can align with cosmological observations within the $\nu$MSM framework. The theoretical implications are profound, suggesting that the $\nu$MSM could singularly provide a unified mechanism for baryogenesis and dark matter production, pending experimental validation, potentially illuminating the path to understanding beyond the Standard Model physics.