Fermion masses and new symmetries beyond the standard model

Abstract

The Standard Model (SM) lacks a satisfactory explanation for the large hierarchy of the fermion masses. The use of extra symmetries is used to create a mechanism to explain this hierarchy.

An additional local U(1) S symmetry is appended to the SM along with an electroweak singlet Higgs boson that has a vacuum expectation value at the electroweak scale. Unlike many other models, the SM fields are neutral under this extra symmetry. Using a Froggatt-Nielsen type mechanism, the mass hierarchy is explained through the interaction of the SM quarks. They do not directly interact with each other, but with additional heavy vector-like quarks that have masses at the TeV scale.

How the Higgs singlet contributes to the generation of the mass hierarchy is generalized across a set of 24 model variation. Some of the phenomenology of Higgs interactions with SM fields by Higgs decays and flavor changing neutral currents are also explored.

The direct interaction of a heavy vector-like quark with a SM quark (the bottom quark) is also examined. The signal rates of this heavy vector-like quark and the SM background are calculated for the dominant signals at the high-energy hadron colliders, multilepton and multi-b-jet final states. For a wide range of masses, the signals are above the background at the LHC.

The neutrino masses are addressed by a different symmetry. Imposing a Z 2 symmetry, the SM neutrinos have interactions with two new Higgs fields and heavy right-handed neutrinos with masses in electroweak scale. The smallness of the left-handed neutrino masses is the result of a combination of fine tuning between the two new Yukawa couplings and using the see-saw mechanism. The proposed scenario gives interesting new Higgs decay signals and can be tested at the LHC.