Leptogenesis from Dark Matter Coannihilation

TL;DR

This work presents a minimal extension of the type-I seesaw model that realizes leptogenesis predominantly from dark-sector coannihilation. By introducing two $Z_2$-odd states, a singlet scalar $\phi$ (DM candidate) and a singlet fermion $\psi$, plus a $Z_2$-even scalar $\eta$, the model links the generation of the baryon asymmetry to the dark matter abundance at TeV scales. Neutrino masses are generated via the standard seesaw with two RHNs and the Casas-Ibarra parametrization, while CP violation arises from both the neutrino Yukawas and the dark-sector couplings, enabling a sizable lepton asymmetry through $\phi\psi \to L H$. Solving the coupled Boltzmann equations shows viable regions where the observed BAU is reproduced together with the correct DM relic, with direct and indirect detection prospects testing the Higgs-portal interactions and resonant effects. The scenario offers a testable, unified explanation for DM and BAU at accessible energies and motivates further collider and astrophysical studies.

Abstract

We propose a minimal extension of the type-I seesaw model to realise leptogenesis from the co-annihilation of dark sector particles. The type-I seesaw model is extended with a singlet fermion and two singlet scalars charged under a $Z_{2}$ symmetry. The $Z_{2}$-odd singlet scalar is the dark matter candidate. Here the usual type-I seesaw mechanism generates neutrino mass, and a net lepton asymmetry is generated from the co-annihilation of the dark matter and the $Z_2$-odd singlet fermion. The $Z_{2}$-even singlet scalar is important in dark matter phenomenology. Successful leptogenesis is possible at TeV-scale, unlike the vanilla case. This minimal extension provides an elegant explanation of successful leptogenesis with direct connection to the dark matter abundance in the Universe.

Figures (19)

• Figure 1: DM coannihilation process responsible for creating the lepton asymmetry.
• Figure 2: Feynman diagrams for self-annihilation of $\phi$ and $\psi$ to $\eta$.
• Figure 3: Feynman diagrams for annihilation of RHNs into $\phi, \psi$.
• Figure 4: Feynman diagrams for washout scattering processes with $\Delta L=1$.
• Figure 5: Feynman diagrams for washout scattering processes with $\Delta L=2$.