text: The mass of the new neutral gauge boson Z ′ arises by the U(1)B  L gauge symmetry 
  − 
  
  breaking, 
  
  M 2 
  Z ′ = 4g2 
  B 
  2. 
  Lv′ 
  (10) 
  − 
  
  Associated with the U(1)B  L gauge symmetry breaking, the RH neutrinos Ni acquire masses 
  − 
  
  v′ 
  √2 
  MNi = 
  (11) 
  
  From LEP experiment, the current lower bound on the Z ′ boson mass has been found to 
  
  be [10, 11] 
  
  MZ ′ 
  gB 
  = 2v′ & 6  7 TeV.  (12) 
  −  L 
  − 
  
  Two Z2-even RH neutrinos N1 and N2 are responsible for light neutrino masses via the 
  
  seesaw mechanism, 
  
  v2 
  2MNi 
  mναβ =  yαiyiβ  .  (13) 
  
  − Xi=1,2 
  Note that the rank of this mass matrix is two, so that the lightest neutrino is massless. 