Search

We extend the standard model with two extra Higgs doublets. Making use of a symmetry principle, we present flavor symmetries based on cycle groups ZN that oblige each Higgs doublet to contribute to the mass of only one generation. The Higgs doublets couple to the fermions with different strengths and in this way accommodate the quark mass hierarchy. We systematically search for all charge configurations that naturally lead to the alignment in flavor space of the quark sectors, resulting in a Cabibbo-Kobayashi-Maskawa matrix near to the identity, determined by the quark mass hierarchy, and with the correct overall phenomenological features. The minimal realization is by the group Z7. We show that only a limited number of solutions exist and that any accidental global symmetry that may occur together with the discrete symmetry is necessarily anomalous. A phenomenological study of each class of solutions concerning predictions to the flavor-changing neutral current phenomena is also performed; for some solutions, it is possible to obtain realistic quark masses and mixing, while the flavor-violating neutral Higgs are light enough to be accessible at the LHC. [ABSTRACT FROM AUTHOR]

We point out that in the standard model there is meaningful quark mixing even in the extreme chiral (EC) limit, where only the third generation of quarks acquires mass. This mixing is in general expected to be of order 1 and the fact that |V13|² + |V23|² ≈ 1.6 × 10-3 implies a novel fine-tuning problem in the SM which we point out for the first time. We propose a possible way of avoiding this fine-tuning by introducing a symmetry S which leads to V CKM = 1, with only the third generation of quarks acquiring mass. We consider two scenarios for generating the mass of the first two quark generations and full quark mixing based on the assumption that the masses of the first two quark families are not generated by the standard Higgs. One consists of the introduction of a second Higgs doublet which is neutral under S. The second scenario consists of assuming new physics at a high energy scale, contributing to the masses of light quark generations, in an effective field theory approach. This last scenario leads to couplings of the Higgs particle to s ¯ s and c ¯ c which are significantly enhanced with respect to those of the SM. In both schemes, one has scalar-mediated flavor-changing neutral currents which are naturally suppressed. Flavor-violating top decays are predicted in the second scenario at the level Br (t → h c)≥ 5× 10-5. [ABSTRACT FROM AUTHOR]