Symmetries, Large Leptonic Mixing and a Fourth Generation

Abstract

We show that large leptonic mixing occurs most naturally in the framework of the Sandard Model just by adding a fourth generation. One can then construct a small $Z_4$ discrete symmetry, instead of the large $S_{4L}\times S_{4R}$, which requires that the neutrino as well as the charged lepton mass matrices be proportional to a $4\times 4$ 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.

6 pages, ReVTeX, no figures

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• Review of Properties of the Top Quark from Measurements at the Tevatron
• CP asymmetry in the decays B --> (X_s, X_d) mu+ mu- with four generations
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• A Simple Approach to Fourth Generation Effects in $B\to X_s{\ell }^{+}{\ell }^{-}$ Decay
• Neutrino Masses and a Fourth Generation of Fermions
• eV Seesaw with Four Generations