Freeze-in leptogenesis without the need for right-handed neutrino oscillations

Abstract

We present an experimentally testable leptogenesis mechanism based on the standard type-I seesaw model that successfully operates at right-handed-neutrino (RHN) masses around the GeV scale. The mechanism takes place in a cosmological background with an asymmetry between right-handed electrons and left-handed positrons generated at high temperatures, and does not require oscillations between RHNs or any CP violation in the RHN sector. In contrast to standard leptogenesis via freeze-in, our mechanism works even in the presence of a single RHN around the GeV scale. The mechanism is illustrated for the minimal type-I seesaw with two RHNs, where we show that successful baryogenesis via leptogenesis is viable in large regions of parameter space even without a small mass splitting between the RHNs.

Figures (5)

• Figure 1: Example solutions of the set of evolution equations in Eqs. \ref{['eq:QKE1']}-\ref{['eq:QKE2']} and \ref{['eq:beq_eR']}. The left (right) panel was obtained with $M=1\, (1)$ GeV, $\delta M=0.5\, (0.5)$, ${\rm Re}\, \omega=\pi/4\, (\pi/4)$, ${\rm Im}\,\omega=1.6\, (5)$, and $\alpha_{31}=3\pi\, (3\pi)$. Solid / dashed lines denote positive / negative asymmetries. The vertical black-dotted curve indicates the right-handed electron equilibration temperature, and the horizontal dotted curve shows the value of $Y_{B-L}$ consistent with the observed BAU Planck:2018vygParticleDataGroup:2024cfk.
• Figure 2: Regions of parameter space where the BAU can be fully accounted for by WIFI-LG in the minimal type-I seesaw model for RHN-mass splitting $\delta M=0.5,{\rm Re}\, \omega=\pi/4$ and initial conditions respecting the bound in Eq. \ref{['eq:upperbound']}. The green, red, and blue curves display expected sensitivities of SHiP ($\lvert{U_{\mu}}\rvert^2$) SHiP:2018xqw, HL-LHC ($\lvert{U_{\mu}}\rvert^2$) Drewes:2019fou, and FCC-ee ($\lvert{U_{\mu}}\rvert^2$) Drewes:2025ocf, respectively. The experimental sensitivities presented here are intended merely as a qualitative guide, as they apply only for the mixing $\lvert{U_{\mu}}\rvert^2$, and the large mass splitting, $\delta M=0.5$, requires a more careful treatment of the associated constraints. See main text and Appendix \ref{['appendix:B']} for further details.
• Figure 3: Regions of parameter space where the BAU can be fully accounted for by WIFI-LG in the minimal type-I seesaw model for RHN-mass splitting $\delta M=0.5,$${\rm Re} \, \omega=\pi/4$ and initial conditions respecting the bound in Eq. \ref{['eq:upperbound']} for normal (left) and inverted (right) neutrino-mass ordering. Allowing $\delta M$ to vary freely, the observed BAU can be explained through pure FILG in the parameter region enclosed by the purple curves, taken from Ref. Klaric:2020phc. The green, red, and blue curves display expected sensitivities of SHiP ($\lvert{U_{\mu}}\rvert^2$) SHiP:2018xqw, HL-LHC ($\lvert{U_{\mu}}\rvert^2$) Drewes:2019fou, and FCC-ee ($\lvert{U_{\mu}}\rvert^2$) Drewes:2025ocf, respectively.
• Figure 4: Time evolution of asymmetries for pure FILG (left) and pure WIFI-LG (right). The values for $\delta M$ and $\mu_{e_R}^0$ are indicated in the panels, and the remaining neutrino parameters are fixed as $M=3GeV,$$\alpha_{31}=3\pi$, ${\rm Re}\,\omega=\pi/4$, and ${\rm Im}\, \omega=0\ (4.9)$ in the top (bottom) panels.
• Figure 5: White: Region of $\delta M$-Im$\omega$ plane where the BAU can be fully accounted for by WIFI-LG in the minimal type-I seesaw model with normal neutrino-mass ordering for $M=10GeV$, ${\rm Re}\, \omega = \pi/4$, and $\alpha_{31}=-\pi\,(+\pi)$ left (right). The purple curve shows the region where pure FILG can account for the BAU.