Freeze-in dark matter via a light Dirac neutrino portal

Abstract

We propose a scenario where dark matter (DM) can be generated nonthermally due to the presence of a light Dirac neutrino portal between the standard model (SM) and dark sector particles. The SM is minimally extended by three right-handed neutrinos (${\ensuremath{\nu}}_{R}$), a Dirac fermion DM candidate ($\ensuremath{\psi}$) and a complex scalar ($\ensuremath{\phi}$), transforming nontrivially under an unbroken ${\mathbb{Z}}_{4}$ symmetry while being singlets under the SM gauge group. While DM and ${\ensuremath{\nu}}_{R}$ couplings are considered to be tiny in order to be in the nonthermal or freeze-in regime, $\ensuremath{\phi}$ can be produced either thermally or nonthermally depending upon the strength of its Higgs portal coupling. We consider both these possibilities and find out the resulting DM abundance via freeze-in mechanism to constrain the model parameters in the light of Planck 2018 data. Since the interactions producing DM also produce relativistic ${\ensuremath{\nu}}_{R}$, we check the enhanced contribution to the effective relativistic degrees of freedom $\mathrm{\ensuremath{\Delta}}{\mathrm{N}}_{\mathrm{eff}}$ in view of existing bounds as well as future sensitivities. We also check the stringent constraints on free-streaming length of such freeze-in DM from structure formation requirements. Such constraints can rule out DM mass all the way up to $\mathcal{O}(100\text{ }\text{ }\mathrm{keV})$ keeping the $\mathrm{\ensuremath{\Delta}}{\mathrm{N}}_{\mathrm{eff}}\ensuremath{\le}\mathcal{O}({10}^{\ensuremath{-}3})$ out of reach from near future experiments. Possible extensions of this minimal model can lead to observable $\mathrm{\ensuremath{\Delta}}{\mathrm{N}}_{\mathrm{eff}}$ which can be probed at next generation experiments.