Linear seesaw leptogenesis before/after electroweak symmetry breaking

TL;DR

A central problem is generating the baryon asymmetry of the Universe within the linear seesaw framework using TeV-scale sterile neutrinos, despite the usual need for heavy right-handed states. The authors propose two natural mechanisms—renormalization group evolution (RGE) and electroweak symmetry breaking (EWSB)—to induce tiny mass splittings within sterile Dirac pairs, triggering resonant CP violation at low scales. They show that RGE-generated splittings can reproduce the observed $Y_B$ for normal and inverted ordering and are compatible with charged lepton flavor violation constraints; EWSB-induced splittings can produce $Y_B$ if leptogenesis occurs before sphaleron freeze-out, and can yield a sizeable lepton asymmetry $Y_L$ after freeze-out, potentially addressing EMPRESS hints. Together, these results establish low-scale linear seesaw leptogenesis as a testable, self-contained mechanism for both neutrino masses and the baryon asymmetry.

Abstract

The linear seesaw (LSS) model provides a natural framework for generating small neutrino masses at low energy scales, thereby offering promising testability prospects. However, in generic LSS models, the exact mass degeneracy (before the electroweak symmetry breaking) between the two sterile neutrinos that form a Dirac pair precludes the generation of CP asymmetries from their interplay, posing a significant challenge to explaining the observed baryon (or lepton) asymmetry of the Universe via the leptogenesis mechanism. In this work, we explore two well-motivated approaches to generate a suitable mass splitting for the two sterile neutrinos that form a Dirac pair, and consequently naturally realize a resonantly enhanced generation of baryon (and lepton) asymmetry. First, we demonstrate that the renormalization group evolution effects can naturally induce the desired mass splitting for the sterile neutrinos, resulting in a successful generation of the observed baryon asymmetry of the Universe. Second, motivated by the recent result from the EMPRESS collaboration that indicates the possible existence of a large lepton asymmetry of the Universe, we explore the possibility that a large lepton asymmetry might naturally follow from the electroweak symmetry breaking which automatically induces the desired mass splitting for the sterile neutrinos.

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 ${\cal O}(M^{}_{\rm LS})$ for some benchmark values of $M^{}_1$. 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 ${\cal O}(M^{}_{\rm LS})$ 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 scenario studied in section 4.1 that only the EWSB contributes to the mass splittings for the sterile neutrinos, in the NO (a) and IO (b) cases, the allowed values of $Y^{}_{\rm B}$ as functions of ${\cal O}(M^{}_{\rm LS})$ for the benchmark value of $M^{}_1=150$ GeV. The horizontal line stands for the observed value of $Y^{}_{\rm B}$.
• Figure 4: For the scenario studied in section 4.1 that only the EWSB contributes to the mass splittings for the sterile neutrinos, 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 ${\cal O}(M^{}_{\rm LS})$ in the NO (a) and IO (b) cases. The horizontal line stands for the current experimental limit on ${\rm BR}(\mu \to e \gamma)$.
• Figure 5: For the scenario studied in section 4.1 that both the RGE effects and the EWSB contribute to the mass splittings for the sterile neutrinos, the allowed values of $Y^{}_{\rm B}$ as functions of ${\cal O}(M^{}_{\rm LS})$ in the NO (a) and IO (b) cases. The horizontal line stands for the observed value of $Y^{}_{\rm B}$.