Non-Thermal Leptogenesis in the BLSM with Inverse Seesaw Mechanism

TL;DR

The paper shows that thermal leptogenesis fails in the gauged $U(1)_{B-L}$ inverse seesaw due to large Yukawa-induced washout at TeV scales. It proposes a non-thermal mechanism where right-handed neutrinos are produced from the decay of the $B-L$ Higgs scalar, with a reheating temperature $T_R$ below the heavy-neutrino mass and enhanced CP violation from the quasi-degenerate heavy neutrinos. A threshold mass relation $M_ ext{chi} o 2 M_N + ext{delta}$ suppresses washout by lowering $T_R$, while resonant CP violation boosts the asymmetry to the observed level; numerical results for $M_N o 5$ TeV show $T_R o 1$ TeV and $ ext{epsilon}_{CP} o obreak 0.48$ can reproduce $ ext{eta}_B^{obs}$. This framework provides a TeV-scale, testable connection between neutrino mass generation and baryogenesis, maintaining predictivity and shedding light on early-Universe dynamics through a concrete particle-physics model.

Abstract

We investigate the viability of non-thermal leptogenesis in the gauged $U(1)_{B-L}$ extension of the Standard Model (BLSM) with an inverse seesaw (ISS) mechanism for neutrino mass generation. In this framework, right-handed neutrinos typically have $\mathcal{O}(1)$ Yukawa couplings, which induce strong washout effects and render conventional thermal leptogenesis ineffective. We demonstrate that a successful baryogenesis scenario can nevertheless be realized through non-thermal leptogenesis, where right-handed neutrinos are produced from the decay of the heavy $B\!-\!L$ Higgs boson $χ$. We explicitly analyze the interplay between the dilution factor $T_R/M_χ$ and the washout parameter characteristic of the ISS, highlighting the tension between suppressing washout effects and maintaining sufficient reheating. We show that a viable lepton asymmetry can be generated provided the scalar mass spectrum is appropriately tuned, allowing for a reduced reheating temperature while keeping washout under control. The resulting lepton asymmetry is efficiently converted into the observed baryon asymmetry of the Universe via sphaleron processes. Our results establish that the inverse-seesaw $B\!-\!L$ model remains a predictive and robust framework for non-thermal leptogenesis and baryogenesis.

Figures (5)

• Figure 1: Parameter combinations $(Y_N,\delta)$ that satisfy $T_R=1~\text{TeV}$ for $M_N=5~\text{TeV}$ in the threshold regime $M_\chi\simeq 2M_N+\delta$. The dark-blue line represents the locus of points yielding the target reheating temperature.
• Figure 2: Illustration of the parameter space consistent with the observed baryon asymmetry in the threshold non-thermal scenario. The dashed black curve tracks the values of $(y_\nu,\delta)$ that reproduce $\eta_B^{\rm obs}$, assuming a resonant choice of the pseudo-Dirac splitting (set by $\mu_S$) that maximizes $\epsilon_{\rm CP}$ for $M_N=5~\text{TeV}$.
• Figure 3: Logarithmic dependence of the resonant mass splitting $\mu_{\rm res}$ required for maximal CP violation as a function of the heavy neutrino mass $M_N$, assuming $m_\nu = 0.05$ eV. The highlighted points correspond to $M_N = 2$, $5$, and $10$ TeV.
• Figure 4: Evolution of the baryon asymmetry $\eta_B$ and effective washout $K_{\rm eff}$ as a function of the reheating temperature $T_R$ for $y_\nu = 10^{-2}$.
• Figure 5: Contours of $\log_{10}(\eta_B)$ in the $(\delta, y_\nu)$ plane. Vertical dotted lines indicate reheating temperature contours. The shaded region corresponds to the non-thermal regime $T_R \ll M_N$, where successful leptogenesis is achieved.