text: 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 Z2 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 
  
  L Higgs VEV v′ & 3 TeV. Thus, we obtain Higgs portal DM 
  by the inverse square of the B 
  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 