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Leptogenesis from a GeV Seesaw without Mass Degeneracy

Marco Drewes, Bjorn Garbrecht

TL;DR

This paper shows that GeV-scale sterile neutrinos can generate the baryon asymmetry of the universe through flavoured leptogenesis without requiring near-degenerate masses. Using nonequilibrium quantum field theory (CTP formalism), the authors derive CP-violating source terms that are flavour-moff-diagonal in the active lepton sector and identify a dominant purely flavoured contribution enhanced by finite-temperature effects. They provide analytic expressions for the flavoured asymmetries and demonstrate, with Casas-Ibarra-based parameter scans and two benchmark scenarios, that viable BAU can arise without the traditional lepton-number–violating mass degeneracy. The work highlights a mechanism (ARS-like flavoured leptogenesis) by which GeV-scale sterile neutrinos evade the usual $M_{ii} > 10^9$ GeV bound and opens a pathway to testable implications in extended Higgs sectors or related models.

Abstract

For Leptogenesis based on the type-I seesaw mechanism, we present a systematic calculation of lepton-number violating and purely flavoured asymmetries within nonequilibrium Quantum Field Theory. We show that sterile neutrinos with non-degenerate masses in the GeV range can explain the baryon asymmetry of the Universe via flavoured Leptogenesis. This is possible due to the interplay of thermal and flavour effects. Our approach clarifies the relation between Leptogenesis from the oscillations of sterile neutrinos and the more commonly studied scenarios from decays and inverse decays. We explain why lower mass bounds for non-degenerate sterile neutrinos derived for Leptogenesis from out-of-equilibrium decays do not apply to flavoured Leptogenesis with GeV-scale neutrinos.

Leptogenesis from a GeV Seesaw without Mass Degeneracy

TL;DR

This paper shows that GeV-scale sterile neutrinos can generate the baryon asymmetry of the universe through flavoured leptogenesis without requiring near-degenerate masses. Using nonequilibrium quantum field theory (CTP formalism), the authors derive CP-violating source terms that are flavour-moff-diagonal in the active lepton sector and identify a dominant purely flavoured contribution enhanced by finite-temperature effects. They provide analytic expressions for the flavoured asymmetries and demonstrate, with Casas-Ibarra-based parameter scans and two benchmark scenarios, that viable BAU can arise without the traditional lepton-number–violating mass degeneracy. The work highlights a mechanism (ARS-like flavoured leptogenesis) by which GeV-scale sterile neutrinos evade the usual GeV bound and opens a pathway to testable implications in extended Higgs sectors or related models.

Abstract

For Leptogenesis based on the type-I seesaw mechanism, we present a systematic calculation of lepton-number violating and purely flavoured asymmetries within nonequilibrium Quantum Field Theory. We show that sterile neutrinos with non-degenerate masses in the GeV range can explain the baryon asymmetry of the Universe via flavoured Leptogenesis. This is possible due to the interplay of thermal and flavour effects. Our approach clarifies the relation between Leptogenesis from the oscillations of sterile neutrinos and the more commonly studied scenarios from decays and inverse decays. We explain why lower mass bounds for non-degenerate sterile neutrinos derived for Leptogenesis from out-of-equilibrium decays do not apply to flavoured Leptogenesis with GeV-scale neutrinos.

Paper Structure

This paper contains 5 sections, 33 equations, 2 figures, 1 table.

Table of Contents

  1. Introduction
  2. Flavoured and Unflavoured Source Terms when $|M_{ii}^2-M_{jj}^2|\gg|\mathbf k|\Gamma_{Ni}$
  3. Lepton Asymmetry and Mass Degeneracy
  4. Flavoured Leptogenesis without Mass Degeneracy
  5. Discussion and Conclusions

Figures (2)

  • Figure 1: In (A) the asymmetry $q_{e}/(s\times2.5\times 10^{-10})$ and in (B) the washout $\Gamma_e/H_{\rm EW}$ of the electron-flavour in Scenario I. The result has the period $\pi$ in ${\rm Re}[\omega_{23}]$. Successful Baryogenesis via Leptogenesis occurs when large negative asymmetries coincide with small washout rates, as in the bottom corners of the diagrams.
  • Figure 2: In (A) the asymmetry $q_{e}/(s\times2.5\times 10^{-10})$ and in (B) the washout $\Gamma_e/H_{\rm EW}$ of the electron-flavour in Scenario II. The result has the period $2\pi$ in ${\rm Re}[\omega_{23}]$. Successful Baryogenesis via Leptogenesis occurs when large negative asymmetries coincide with small washout rates, as in the bottom corners of the diagrams.