text: and mass spectrum of the Higgs bosons. 
  
  The thermal relic abundance of DM 
  
  109 mN /Td 
  √g 
  σv 
  MP h 
  
  ΩN h2 = 1.1  1,  GeV−  (14) 
  × 
  i 
  
  with the Planck mass MP , the thermal averaged product of the annihilation cross section 
  
  σv  and the relative velocity  , the total number of relativistic degrees of freedom in the 
  h  i 
  
  thermal bath g  , and the decoupling temperature Td, is evaluated by solving the Boltzmann 
  ∗ 
  
  equation for the number density of RH neutrino nN ; 
  
  dnN 
  dt 
  (n2 
  n2 
  EQ), 
  + 3HnN =  σv 
  N −  −h  i 
  
  and the Friedmann equation 
  
  2 
  
  ˙a 
  a(cid:19) 
  8π 
  3M 2 
  P 
  = 
  ≡ (cid:18) 
  
  with nEQ and a(t) being the equilibrium number density and the scale factor, under the 
  
  radiation dominated Universe with the energy density ρ = ρrad [14]. 
  
  5 