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Discrete Family Symmetries for Quarks and Leptons and A Novel Origin of CP Violation

HEP Division Seminar
Mu-Chun Chen, University of California, Irvine
March 26, 2014 11:00AM to 12:00PM
Building 362, Room F108
The Standard Model has been tested experimentally to a high degree of accuracy. Despite its successes in describing particle interactions at very short distances, it has many glaring deficiencies. Among these are the many free parameters that parametrize fermion masses and mixings. The origin of fermion mass hierarchy and mixing still remains one of the great mysteries in particle physics. Even though the fermion masses are generated by the Higgs mechanism, the Higgs mechanism by itself does not explain the observed mass hierarchy and mixing patterns.

The discovery of non-zero neutrino masses leads to yet another puzzle: why the neutrino masses are so small when compared to other fermions, and why two of the three neutrino mixing angles are so large when compared with their quark counterpart. Furthermore, CP violation in the Standard Model is insufficient to explain the observed cosmological baryon number asymmetry. On the other hand, the recent observation of a large value for the third neutrino mixing angle implies good future experimental prospects for discovering a new source of CP violation in the neutrino sector. This new CP violation source may be relevant for the dynamical generation of the cosmological matter-antimatter asymmetry.

In this talk, I will discuss how these outstanding questions in particle physics can be addressed by new physics beyond the Standard Model. In particular, I will focus on models based on grand unification in combination with discrete family symmetries which give rise to realistic masses and mixing angles of all observed fermions, including the neutrinos, with a significantly reduced number of parameters. I will point out a novel interesting possibility that for certain discrete symmetries, CP violation can be entirely group theoretical in origin. This leads to interesting experimental predictions as well as implications for the generation of the matter-antimatter asymmetry in the Universe.