Does one need a fourth generation of leptons?

Abstract

Massive neutrinos with masses O(eV) and O(keV) can be naturally incorporated in the standard model with only left-handed neutrinos by assuming the existence of a fourth lepton family and of an approximate global symmetry that keeps the neutrino masses almost degenerate in pairs. Besides the existence of a new heavy charged lepton, the model gives interesting predictions for future neutrino experiments both at accelerators and underground.