Asymmetric Dark Matter from Low-Scale Spontaneous Leptogenesis

TL;DR

This paper introduces a unified asymmetric dark matter framework where both the baryon asymmetry and the dark matter asymmetry arise from low-scale spontaneous leptogenesis driven by a dynamical majoron background. The key mechanism is a derivative coupling from the majoron that yields CPT violation, enabling inverse decays of the lightest right-handed neutrino to generate both asymmetries with a shared CP phase, $\dot{\theta}$. A predictive relation for the dark matter mass emerges, with freeze-out yielding $m_{\chi}$ in the sub-100 GeV range and freeze-in extending up to tens of TeV, depending on the Yukawa couplings $y_{N,1}$ and $y_{\chi,1}$. The model also discusses phenomenological constraints from majoron dynamics, dark-photon annihilation, and cosmology, and highlights potential direct-detection prospects in the freeze-out scenario. Overall, it provides a coherent link between leptogenesis, DM genesis, and observable DM mass scales under low-scale dynamics.

Abstract

We investigate a novel type of asymmetric dark matter (ADM) model in which the dark matter asymmetry and the baryon asymmetry in our universe (BAU) are produced simultaneously via low-scale spontaneous leptogenesis, where the mass scale of right-handed neutrino is much lower than the Davidson-Ibarra bound $M_1 \ll 10^{9}~\rm{GeV}$. In our scenario, both asymmetries are predominantly sourced by a dynamical $CP$ phase, namely the majoron. Its kinetic misalignment provides a sufficiently large, time-dependent effective $CP$ phase, allowing efficient asymmetry production even for low-scale right-handed neutrinos. In our framework, the sources of $CP$ violation responsible for the BAU and ADM are correlated with each other, leading to a predictive relation for the dark matter mass. In particular, when the dark matter asymmetry reaches its equilibrium value before freeze-out, the dark matter mass is typically predicted to lie in the range $\mathcal{O}(0.1)~\mathrm{GeV} \lesssim m_χ \lesssim \mathcal{O}(100)~\mathrm{GeV}$, which lies within the sensitivity of direct detection experiments. On the other hand, if the dark matter asymmetry does not reach its equilibrium value due to weak coupling, the allowed mass range extends over a broader interval, $\mathcal{O}(0.1)~\mathrm{GeV} \lesssim m_χ \lesssim \mathcal{O}(10)~\rm{TeV}$.

Figures (7)

• Figure 1: A schematic view of our framework: the right-handed neutrinos are coupled to the SM leptons as well as to the DM particle through Yukawa interactions. In the early universe, when a dynamical $CP$ phase is present, the inverse decay processes simultaneously generate SM lepton asymmetry and DM asymmetry.
• Figure 2: Evolution of the baryon asymmetry generated through inverse decay in the presence of a majoron background. The mass and Yukawa coupling of the right-handed neutrino are fixed at $(M_1, y_{N,1}) = (3 \times 10^5 ~{\rm GeV}, 2 \times 10^{-5})$, which yields $K \simeq 50$. To reproduce the observed baryon asymmetry, we set $Y_\theta \simeq 10^{-7} g_{N,1}^{-2} (z_{\mathrm{fo}}^L/10)^2$.
• Figure 3: The interaction rates of $L H \to N_1$, $\chi\phi \to N_1$, and $\chi\phi\to\bar{\chi}\phi^*$, and the Hubble rate are shown as functions of $z$.
• Figure 4: Evolution of the dark matter asymmetry generated through inverse decay in the presence of a majoron background, shown for different values of $|y_{\chi,1}|$. The mass and Yukawa coupling of the right-handed neutrino, as well as the value of $Y_\theta$, are taken to be the same as in Fig. \ref{['fig: Solution for BAU Boltzmann equation']}. The final value of $Y_{\Delta \chi}$ reaches its maximum when $|y_{\chi,1}|$ lies at the boundary between the freeze-out and freeze-in regimes of the inverse decay. In the freeze-out regime, larger $|y_{\chi,1}|$ leads to a smaller $Y_{\Delta \chi}$, while in the freeze-in regime, smaller $|y_{\chi,1}|$ results in a smaller $Y_{\Delta \chi}$.
• Figure 5: The dark matter mass $m_{\chi}$ required to account for the observed relic abundance in the freeze-out scenario. As benchmark points, we choose $(M_1, y_{N,1}) = (3 \times 10^3~{\rm GeV}, 2.2 \times 10^{-6})$ (black solid line), $(3 \times 10^4~{\rm GeV}, 7 \times 10^{-6})$ (dashed line), and $(3 \times 10^5~{\rm GeV}, 2 \times 10^{-5})$ (dotted line).