[Paper Review] Recent Issues in Leptogenesis

Enrico Nardi|ArXiv.org|Jun 4, 2007

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TL;DR

This paper reviews recent advances in leptogenesis, emphasizing the critical roles of lepton flavor effects and contributions from heavier right-handed neutrinos $N_{2,3}$ in generating the baryon asymmetry of the Universe. It demonstrates that initial asymmetries from $N_{2,3}$ decays can survive $N_1$-mediated washout processes, invalidating the common assumption that $N_1$ leptogenesis is independent of initial conditions in the strong washout regime, with implications for neutrino mass bounds and model constraints.

ABSTRACT

Baryogenesis via leptogenesis provides an appealing mechanism to explain the observed baryon asymmetry of the Universe. Recent refinements in the understanding of the dynamics of leptogenesis include detailed studies of the effects of lepton flavors and of the role possibly played by the lepton asymmetries generated in the decays of the heavier singlet neutrinos $N_{2,3}$. A review of these recent developments in the theory of leptogenesis is presented.

Motivation & Objective

• To re-express and clarify the theoretical framework of leptogenesis, particularly the role of lepton flavor and heavier neutrinos $N_{2,3}$ in baryogenesis.
• To challenge the long-standing assumption that $N_1$ leptogenesis is insensitive to initial conditions in the strong washout regime.
• To quantify how pre-existing asymmetries from $N_{2,3}$ decays can survive $N_1$-mediated washout processes.
• To reassess constraints on low-energy neutrino parameters, especially the absolute neutrino mass scale, in light of these new dynamics.
• To correct and extend prior analyses by incorporating overlooked effects such as flavor structure, Higgs asymmetries, and spectator reactions.

Proposed method

• Formulates the leptogenesis Lagrangian including $N_1$, $N_2$, and $N_3$ with Yukawa couplings to lepton doublets and the Higgs field.
• Applies Boltzmann equations to track the evolution of lepton and Higgs number densities, incorporating washout processes and $CP$-violating asymmetries.
• Uses a flavor basis where lepton states are defined via the Yukawa coupling matrix $\lambda\lambda^\dagger$, enabling treatment of non-orthogonal flavor states.
• Analyzes the survival of asymmetries in orthogonal subspaces (e.g., $\ell_{1\perp}$) during $N_1$ washout, showing protection from $N_1$-induced processes.
• Considers thermal and non-thermal regimes, distinguishing between $T \gtrsim 10^{12}$ GeV (flavor-irrelevant) and $T \lesssim 10^9$ GeV (fully resolved flavor structure).
• Evaluates the impact of spectator reactions and sphaleron processes on asymmetry evolution, particularly in the context of $B+L$ violation and $B$-conserving interactions.

Experimental results

Research questions

• RQ1 To what extent can lepton asymmetries generated by $N_{2,3}$ decays survive $N_1$-mediated washout processes in the strong washout regime?
• RQ2 How do lepton flavor effects alter the standard picture of $N_1$-only leptogenesis and its dependence on initial conditions?
• RQ3 What constraints do these effects place on the absolute neutrino mass scale, particularly in light of the $m_\nu \gtrsim 0.2$ eV bound?
• RQ4 In what parameter regimes does the assumption of $N_1$-leptogenesis independence from initial conditions break down?
• RQ5 How do thermal corrections, Higgs asymmetries, and spectator reactions influence the final baryon asymmetry in multi-singlet models?

Key findings

• Asymmetries in lepton states orthogonal to $\ell_1$ (e.g., $\ell_{1\perp}$) are protected from $N_1$-mediated washout, allowing them to survive and contribute to the final baryon asymmetry.
• The sum $Y_{\ell_1} + Y_H$ vanishes under strong $N_1$ washout, but $Y_{\ell_1}$ and $Y_H$ do not individually vanish, preserving a component of the asymmetry.
• When $M_1 \lesssim 10^9$ GeV, full flavor resolution occurs, and no asymmetry direction is protected, leading to complete erasure of pre-existing asymmetries.
• The common assumption that $N_1$ leptogenesis is independent of initial conditions fails in general, except in three specific cases: vanishing $\epsilon_2\eta_2$ and $\epsilon_3\eta_3$, $N_1$ washouts remaining significant at $T \lesssim 10^9$ GeV, or reheating between $M_2$ and $M_1$.
• Flavor effects can modify the final baryon asymmetry by up to an order of magnitude, invalidating prior constraints that assumed $N_1$-only dynamics.
• The limit on the absolute neutrino mass scale ($m_\nu \gtrsim 0.2$ eV) previously thought to rule out standard leptogenesis is weakened, as $N_{2,3}$-induced asymmetries can compensate for suppressed $N_1$ contributions.