Light Dirac neutrino portal dark matter with observable ΔN eff

Abstract

We propose a Dirac neutrino portal dark matter scenarioby minimally extending the particle content of the Standard Model(SM) with three right-handed neutrinos (ν R ), a Dirac fermiondark matter candidate (ψ) and a complex scalar (ϕ), allof which are singlets under the SM gauge group. An additional ℤ4symmetry has been introduced for the stability of dark mattercandidate ψ and also ensuring the Dirac nature of lightneutrinos at the same time. Both the right handed neutrinos andthe dark matter thermalise with the SM plasma due to a new Yukawainteraction involving ν R , ψ and ϕ while the lattermaintains thermal contact via the Higgs portal interaction.The decoupling of ν R occurs when ϕloses its kinetic equilibrium with the SM plasma and thereafterall three ℤ4 charged particles form an equilibriumamong themselves with a temperature TνR .The dark matter candidate ψ finally freezes out within the darksector and preserves its relic abundance. We have found that in thepresent scenario, some portion of low mass dark matter (M ψ ≲ 10 GeV)is already excluded by the Planck 2018 data for keeping ν R sin the thermal bath below a temperature of 600 MeV and therebyproducing an excess contribution to N eff. The nextgeneration experiments like CMB-S4, SPT-3G etc. will have the required sensitivities to probe the entiremodel parameter space of this minimal scenario,especially the low mass range of ψ where direct detectionexperiments are still not capable enough for detection.