Light Dirac neutrino portal dark matter with observable $\Delta{N_{\rm
  eff}}$
  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 ($\nu_R$), a Dirac fermion dark matter candidate ($\psi$) and
a complex scalar ($\phi$), 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 $\psi$ 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
$\nu_R$, $\psi$ and $\phi$ while the latter maintains thermal contact via the
Higgs portal interaction. The decoupling of $\nu_R$ occurs when $\phi$ 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 $T_{\nu_R}$. The dark matter candidate $\psi$ 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
($M_{\psi}\lesssim10$ GeV) is already excluded by the Planck 2018 data for
keeping $\nu_R$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 $\psi$ where direct detection experiments are still not
capable enough for detection.
