Rescuing leptogenesis in inverse seesaw models with the help of non-Abelian flavor symmetries

TL;DR

The paper tackles the challenge of achieving successful leptogenesis in inverse seesaw models with hierarchical pseudo-Dirac sterile-neutrino pairs by invoking non-Abelian flavor symmetries that naturally enforce degeneracies among three PD pairs and then break them to enable resonant leptogenesis across pairs. It analyzes two breaking mechanisms: (i) renormalization-group evolution (RGE) between a high flavor-symmetry scale and the sterile-neutrino scale, which induces mass splittings via differences in $Y_\nu^agger Y_\nu$, and (ii) a nontrivial flavor structure in the $μ_s$ matrix that breaks degeneracies directly; both pathways are examined with and without sizable RGE effects. The study provides detailed formulae for the ISS mass matrices, CP asymmetries, and washouts in a multi-PD-state resonant framework, and presents numerical results showing that the observed BAU $Y_B$ can be reproduced for TeV-scale $M_0$, with viable parameter regions also testable via charged-lepton flavor-violating processes such as $\mu \\to e \\gamma$. The findings demonstrate that flavor symmetries offer a robust route to rescue leptogenesis in ISS, yielding testable predictions and a broader viable parameter space than in naive implementations.

Abstract

The inverse seesaw (ISS) model provides an attractive framework that can naturally explain the smallness of neutrino masses while accommodating some sterile neutrinos potentially accessible at present or future experiments. However, in generic ISS models with hierarchical pseudo-Dirac (PD) sterile neutrino pairs, the generation of the observed baryon asymmetry of the Universe via the leptogenesis mechanism is extremely challenging. In this paper, we investigate rescuing leptogenesis in the ISS model with the help of non-Abelian flavor symmetries which have the potential to explain the observed peculiar neutrino mixing pattern: we first implement non-Abelian flavor symmetries to naturally enforce mass degeneracies among different pseudo-Dirac sterile neutrino pairs and then break them in a proper way so that resonant leptogenesis among different PD sterile neutrino pairs can arise, thus enhancing the generated baryon asymmetry. To be specific, we have considered the following two well-motivated approaches for generating the tiny mass splittings among different PD sterile neutrino pairs: one approach makes use of the renormalization-group corrections to the sterile neutrino masses, while the other approach invokes non-trivial flavor structure of the $μ_{\rm s}$ matrix. For these two scenarios, we aim to explore the viability of leptogenesis and to identify the conditions under which the observed baryon asymmetry can be successfully reproduced.

Figures (6)

• Figure 1: For the scenario studied in section 3.1, in the NO (a) and IO (b) cases, the allowed values of $Y^{}_{\rm B}$ as functions of $\mu_0$ for some benchmark values of $M^{}_0$. The horizontal line stands for the observed value of $Y^{}_{\rm B}$.
• Figure 2: For the scenario studied in section 3.1, in the parameter space that allows for a reproduction of the observed value of $Y^{}_{\rm B}$, the allowed values of ${\rm BR}(\mu \to e \gamma)$ as functions of $\mu_0$ in the NO (a) and IO (b) cases. The horizontal line stands for the current upper bound on ${\rm BR}(\mu \to e \gamma)$.
• Figure 3: For the first scenario studied in section 4 where we only consider the contributions of the non-trivial flavor structure of $\mu_{\rm s}$ to the mass splittings among different PD sterile neutrino pairs, in the NO (IO) case, the allowed values of $Y^{}_{\rm B}$ as functions of $M^{(0)}_{\rm D}$ for some benchmark values of $M^{}_0$. The horizontal line stands for the observed value of $Y^{}_{\rm B}$.
• Figure 4: For the first scenario studied in section 4, in the parameter space that allows for a reproduction of the observed value of $Y^{}_{\rm B}$, the allowed values of ${\rm BR}(\mu \to e \gamma)$ as functions of $M^{(0)}_{\rm D}$ in the NO (a) and IO (b) cases. The horizontal line stands for the current upper bound on ${\rm BR}(\mu \to e \gamma)$.
• Figure 5: For the second scenario studied in section 4 where we take into account both the contributions of the RGE effects and $\mu^{}_{\rm s}$ to the mass splittings among different PD sterile neutrino pairs, in the NO (IO) case, the allowed values of $Y^{}_{\rm B}$ as functions of $M^{(0)}_{\rm D}$ for some benchmark values of $M^{}_0$. The horizontal line stands for the observed value of $Y^{}_{\rm B}$.