Neutrino Mixing via the Neutrino Portal

Relation between the lepton and quark mixings: $U_{PMNS} \approx V_{CKM}^{\dagger} U_X$,where$U_X$ is the BM or TBM mixing matrices, implies the quark-lepton (Grand) unification and existence of hidden sector with certain flavor symmetries. The latter couples to the visible sector via the neutrino portal and is responsible for $U_X$, as well as for smallness of neutrino mass. GUT ensures appearance of $\sim V_{CKM}$ in the lepton mixing. General features of this scenario (inverse or double seesaw, screening of the Dirac structures, basis fixing symmetry) are described and two realizations are presented. The high energy realization is based on $SO(10)$ GUT with the hidden sector at the Planck scale. The low energy realization includes the 100 TeV scale $L-R$ symmetry and the hidden sector at the keV - MeV scale.