Cogenesis of visible and dark matter in type-I Dirac seesaw

Abstract

We propose a novel cogenesis framework based on the type-I Dirac seesaw mechanism. The minimal type-I Dirac seesaw with three heavy vector-like fermions $(N)$, one singlet scalar $(η)$, and the right-handed counterparts $(ν_R)$ of the Standard Model (SM) neutrinos is extended to include a Dirac fermion dark matter (DM) $(χ)$ and its heavier scalar companion ($ϕ$). The out-of-equilibrium decays of the vector-like fermion generate asymmetries simultaneously in the visible sector, through decay channels involving $(ν_R,η)$ or lepton, Higgs doublets in the SM, and in the dark sector via decaying into $(χ,ϕ)$. The resulting lepton asymmetry is partially converted into the observed baryon asymmetry by electroweak sphaleron processes, while the dark-sector asymmetry survives to constitute the present-day asymmetric DM relic. The generation of asymmetries in multiple sectors and their mutual washouts provide rich dynamics while also keeping the model testable at different observations involving DM, neutrinos, cosmic microwave background (CMB), as well as gravitational waves (GW). We find that successful cogenesis can be realized for DM masses in the range $100~\mathrm{MeV} \lesssim m_χ\lesssim 39~\mathrm{TeV}$. The lower bound arises from the requirement that the symmetric component of DM annihilates efficiently before the big bang nucleosynthesis (BBN) epoch, while the upper bound is set by unitarity constraints on the asymmetric DM.