A NATURALLY HEAVY 4TH GENERATION NEUTRINO

We propose that there exists a fourth generation, all neutrinos have Dirac masses of order the corresponding charge lepton partner, and right-handed neutrinos have Majorana masses of order the weak scale. The see-saw mechanism then implies the light neutrino masses are O(mMlepton2/M), while a fourth generation neutrino is heavy with mass O(M)∼O(Mw), and would not have been seen in the Z0 width a LEP. Such a solution appears to be compatile with all known phenomenological and cosmological constraints, and predicts masses for neutrinos near their current upper bounds. © 1990.