Symmetries, large leptonic mixing and a fourth generation

We show that large leptonic mixing occurs most naturally in the framework of the Standard Model just by adding a fourth generation. One can then construct a small Z4 discrete symmetry, instead of the large S4LxS4R, which requires that the neutrino as well as the charged lepton mass matrices be proportional to a 4x4 democratic mass matrix, where all entries are equal to unity. Without considering the see-saw mechanism, or other more elaborate extensions of the SM, and contrary to the case with only 3 generations, large leptonic mixing is obtained when the symmetry is broken. (author)