parameters to be consistent with the current observations. Next we calculate the scattering cross section between the DM particle and a proton and discuss the implication for the direct DM search experiments. The DM RH neutrino interacts with the SM particles through couplings with B − L gauge and B − L Higgs bosons. Note that neutrino Dirac Yukawa interactions are absent because of the Z$_{2}$ parity. The most of annihilation of the RH neutrinos occurs via Z ′ , H and h exchange processes in the s -channel. In practice, the dominant contributions come from the Higgs ( h and H ) exchange diagrams, because the Z ′ exchange processes are suppressed by the inverse square of the B − L Higgs VEV v ′ ≳ 3 TeV. Thus, we obtain Higgs portal DM of RH neutrino effectively. The relevant annihilation modes are the annihilation into f ¯ f , W $^{+}$W − , ZZ , and h ( H ) h ( H ). Since RH neutrino DM couples to only B − L Higgs Ψ while a SM particle does to SM Higgs Φ, the DM annihilation occurs only through the mixing between these two Higgs bosons. Although it is not so severe, the precision electroweak measurements [12] as well as the unitarity bound [13] give constraints on the mixing angle and mass spectrum of the Higgs bosons. The thermal relic abundance of DM with the Planck mass M$_{P}$ , the thermal averaged product of the annihilation cross section and the relative velocity 〈 σv 〉 , the total number of relativistic degrees of freedom in the thermal bath g ∗ , and the decoupling temperature T$_{d}$ , is evaluated by solving the Boltzmann equation for the number density of RH neutrino n$_{N}$ ; and the Friedmann equation with n$_{EQ}$ and a ( t ) being the equilibrium number density and the scale factor, under the radiation dominated Universe with the energy density ρ = ρ$_{rad}$ [14]. A. Thermal relic density Ω$_{N}$ h 2 = 1 . 1 × 10 9 m$_{N}$/T$_{d}$ √ g ∗ M$_{P}$ 〈 σv 〉 GeV − $^{1}$, (14) dn$_{N}$ dt + 3 Hn$_{N}$ = −〈 σv 〉 ( n 2 N − n 2 $_{EQ}$) , (15) H 2 $_{≡}$( ˙ a a ) 2 = 8 π 3 M 2 ρ, (16) 5