Asymmetric long-lived dark matter and leptogenesis from the type-III seesaw framework

We propose a simple model in the type-III seesaw framework to explain the neutrino mass, asymmetric dark matter (DM), and baryon asymmetry of the Universe. We extend the Standard Model with a vectorlike singlet lepton (χ) and a hypercharge zero scalar triplet (Δ) in addition to three hypercharge zero triplet fermions (Σi,i=1, 2, 3). A Z2 symmetry is imposed under which χ and Δ are odd, while all other particles are even. As a result, the lightest Z2 odd particle χ behaves as a candidate of DM. In the early Universe, the CP-violating out-of-equilibrium decay of heavy triplet fermions to the Standard Model lepton (L) and Higgs (H) generate a net lepton asymmetry, while that of triplet fermions to χ and Δ generate a net asymmetric DM. The lepton asymmetry is converted to the required baryon asymmetry of the Universe via the electroweak sphalerons, while the asymmetry in χ remains as a DM relic that we observe today. We introduce a singlet scalar Φ, with mass MΦ<Mχ, which not only assists to deplete the symmetric component of χ through the annihilation process χ¯χ→ΦΦ, but also paves a path to detect DM χ at direct search experiments through Φ−H mixing. The electroweak symmetry breaking induces a nonzero vacuum expectation value to Δ, which leads to an unstable asymmetric DM ranging from a few MeV to hundreds of GeV. We then explore the displaced vertex signatures of the charged components of the scalar triplet Δ at colliders. Published by the American Physical Society 2025