Universal Seesaw Mechanism for Quarks and Leptons

Abstract

Universal seesaw mechanism (USM) is implemented in a left-right symmetric unified theory of extended color and electroweak interactions to solve the problems of fermion mass hierarchies and generation mixings. The gauge group has the structure SU(4)cL ×SU(4)cR ×SU(2)L ×SU(2)R ×U(1)X in which the lepton number is treated as the fourth color and the U(1)X group is generated by a new charge X. Colored and non-colored Higgs fields induce USM between ordinary fermion multiplets and exotic electroweak singlets. USM is singly applied to the charged fermion sectors to suppress their average mass below the electroweak mass scale. On the other hand, USM applied doubly to the neutral fermion sector works to make neutrinos superlight. Prodigious gap between vanishingly small neutrino masses and the 102 GeV scale of the yet-undiscovered top quark is explained without presuming unnaturally huge mass scales. A global horizontal U(1)A symmetry is introduced so as to circumvent the strong CP violation and to restrict the Yukawa coupling constants. Colored Higgs quartets break the extended color symmetry SU(4)cL ×SU(4)cR down to SU(3)cL ×SU(3)cR by making the color interaction of lepton to be superweak. It is three kinds of colored Higgs bi-quartets which degrade the left-right symmetric color interaction into the conventional color interaction of vector type and play at the same time the roles to cause the USM and the generation mixing leading to a realistic variety in each sector of the fermion mass spectrum. Characteristics of mass spectra of charged fermions and the quark mixing matrix are described by adjusting the Yukawa coupling constants over a reasonable range of values.