SEESAW ENHANCEMENT OF LEPTON MIXING

The seesaw mechanism of neutrino mass generation may enhance lepton mixing up to maximal even if the Dirac mass matrices of leptons have a structure similar to that in the quark sector. Two sets of conditions for such an enhancement are found. The first one includes the seesaw generation of heavy Majorana masses for right-handed neutrinos and a universality of Yukawa couplings which can follow from the unification of neutrinos with new superheavy neutral leptons. The second set is related to the lepton number symmetry of the Yukawa interactions in the Dirac basis of neutrinos. Models which realize these conditions have a strong hierarchy or strong degeneration of Majorana masses of the right-handed neutrinos.