Light Dirac neutrino portal dark matter with observable $Δ{N_{\rm eff}}$
We propose a Dirac neutrino portal dark matter scenario by minimally extending the particle content of the Standard Model (SM) with three right handed neutrinos ($ν_R$), a Dirac fermion dark matter candidate ($ψ$) and a complex scalar ($ϕ$), all of which are singlets under the SM gauge group. An additional $\mathbb{Z}_4$ symmetry has been introduced for the stability of dark matter candidate $ψ$ and also ensuring the Dirac nature of light neutrinos at the same time. Both the right handed neutrinos and the dark matter thermalise with the SM plasma due to a new Yukawa interaction involving $ν_R$, $ψ$ and $ϕ$ while the latter maintains thermal contact via the Higgs portal interaction. The decoupling of $ν_R$ occurs when $ϕ$ loses its kinetic equilibrium with the SM plasma and thereafter all three $\mathbb{Z}_4$ charged particles form an equilibrium among themselves with a temperature $T_{ν_R}$. The dark matter candidate $ψ$ finally freezes out within the dark sector and preserves its relic abundance. We have found that in the present scenario, some portion of low mass dark matter ($M_ψ\lesssim10$ GeV) is already excluded by the Planck 2018 data for keeping $ν_R$s in the thermal bath below a temperature of 600 MeV and thereby producing an excess contribution to $N_{\rm eff}$. The next generation experiments like CMB-S4, SPT-3G etc. will have the required sensitivities to probe the entire model parameter space of this minimal scenario, especially the low mass range of $ψ$ where direct detection experiments are still not capable enough for detection.