A See-saw model of sterile neutrino
  If the smallness of the mass of the sterile neutrino is to be explained by the see-saw mechanism, the off-diagonal entries of the mass matrix needs to be protected by some symmetry not far above the electroweak scale. We implement see-saw mechanism in a gauge model based on $SU(2)^q_L \times SU(2)^l_L \times U(1)^q_Y \times U(1)^l_Y$ un-unified gauge group which breaks to $SU(2)_L \times U(1)_Y$ at the TeV region via a two-step symmetry breaking chain. The right handed diagonal block is tied to the highest scale up to which the un-unification symmetry holds. The sterile neutrino emerges from a quark-lepton mixed representation of the un-unified group. 
Biswajoy Brahmachari (Indiana U)