Linking Leptogenesis and Asymmetric Dark Matter: A Testable Framework for Neutrino Mass and the Matter-Antimatter Asymmetry

Abstract

We investigate a minimal extension of the Leptogenesis framework that simultaneously explains the observed baryon asymmetry and dark matter (DM) abundance through the decay of a heavy Majorana neutrino. In this scenario, CP violation arises from complex Yukawa couplings, enabling the generation of asymmetries in both the Standard Model (SM) and DM sectors. We explore two regimes: (i) wash-in, where an initial dark asymmetry is transferred to SM leptons by $2 \leftrightarrow 2$ scattering processes; and (ii) co-genesis, featuring a hierarchical coupling structure that allows enhanced CP violation while supporting a low-scale seesaw mechanism at order $\mathcal{O}(2)$ TeV. This setup not only links light neutrino masses to the Majorana mass term but also suggests that lepton-number violation may occur at experimentally accessible energy scales. In the co-genesis scenario, we show spin-independent cross sections for DM heavier than 10 GeV that can be tested in current direct detection experiments and motivate the exploration of cross sections inside the neutrino fog for lighter DM masses, establishing asymmetric leptogenesis as a predictive benchmark framework for direct-detection experiments and identifying a new hierarchical-coupling regime enabling TeV-scale leptogenesis.

Figures (8)

• Figure 1: Schematic illustration of the two asymmetry-generation mechanisms studied in this work: (left) the wash-in scenario, where an asymmetry first produced in the dark sector is transferred to the Standard Model via scattering, and (right) the co-genesis scenario, where visible and dark asymmetries are generated simultaneously in heavy-neutrino decays, highlighting the dynamical origin of the baryon–dark matter abundance relation.
• Figure 2: Departure of the heavy-neutrino abundance from equilibrium as a function of $z=M_{N_i}/T$, showing numerical (solid) and analytical (dashed) solutions for different $\Gamma_{N_i}/H_i$, where smaller decay rates produce larger departures from equilibrium and therefore more efficient asymmetry generation.
• Figure 3: Effect of increasing the scalar mass ratio $m_\phi/M_{N_1}$ on dark-sector washout, demonstrating that larger $m_\phi$ suppresses late-time $2\leftrightarrow2$ washout processes, allowing a larger dark asymmetry to survive and enabling successful leptogenesis at lower RHN mass scales. The solid lines correspond to BM3 with no dark-washout suppression, where $m_\phi/M_{N_1}=r_{\phi 1}\simeq 0$ and $m_\phi\gtrsim m_{\chi}\simeq 78~\mathrm{GeV}$. The dashed lines correspond to BM4 with dark-washout suppression, where $r_{\phi 1}=0.2$, $m_{\phi}\simeq 1.7\times 10^{6}~\mathrm{GeV}$, and $m_{\chi}\simeq 0.11~\mathrm{GeV}$. The red dot-dashed horizontal line shows the asymptotic analytic estimate of BM3 for dark asymmetry from eqs. (\ref{['eq:saha']}) and (\ref{['eq:asympchi']}).
• Figure 4: Contours of the minimum RHN mass $M_{N_1}$ required to reproduce the observed asymmetry as a function of coupling ratios, illustrating how hierarchical dark-sector couplings lower the leptogenesis scale, with the highlighted point indicating the parameter region achieving the minimal viable mass. Left demonstrates the effect of the ratio $|\lambda_1|/|y_1|$ and $|\lambda_2|$ on the $N_1$ mass bound. $|\lambda_1|/|y_1|\lesssim 1$ has less CP violation, $|\lambda_1|/|y_1|\gtrsim1$ has less departure from thermal equilibrium. Right shows how the bound changes as the hierarchy between $|\lambda_2|$ and $|\lambda_1|$ increases, with the black line showing the minimum $N_1$ mass for the corresponding $\lambda_2$ coupling. The red circle highlights the parameter point where the bound is lowest. The mass gap is set to $M_{N_2}/M_{N_1}\simeq 1.35$, $|y_1|\approx\sqrt{M_{N_1}}\cdot 6.8\times 10^{-10}/\sqrt{[\text{GeV}]}$, $|y_2|\approx\sqrt{M_{N_2}}\cdot6.5\times 10^{-8}/\sqrt{[\text{GeV}]}$.
• Figure 5: Dependence of the dark matter mass $m_\chi$ and the required $M_{N_1}$ on the washout-suppression parameter $r_{\phi 1}$ to reproduce the observed asymmetry, showing that increased washout suppression allows lighter RHN masses while maintaining the observed DM relic abundance. The scalar mass $m_\phi$ is shown since DM stability requires $m_\chi<m_\phi$. The remaining parameters are chosen as explained in the text. The curves meet at $r_{\phi 1}\simeq 0.44$, where the required $M_{N_1}$ is lowest.