| In this dissertation, we investigate a mechanism for leptogenesis at the electroweak scale in a model of dark energy and dark matter proposed by Prof. P. Q. Hung. This model involves an asymptotically free gauge group SU(2)z and an axion-like particle with an SU(2)z-instanton-induced potential which yields a scenario for the dark energy. Furthermore, the extended particle content of this SU(2)z model contains a possible candidate for the cold dark matter, the SU(2) z "shadow" fermion, which couples with the Standard Model lepton through a scalar "messenger field" carrying both SU(2)z and electroweak quantum numbers. Since these shadow fermions are in a real representation of SU(2) z, lepton number can be violated in the Yukawa sector and a lepton number asymmetry can be generated in the SU(2) z particle's decay which is also CP-violating and "out-of-equilibrium". We computed the asymmetry coming from the interference between the tree-level and one-loop amplitudes for both messenger scalar and shadow fermion decays. From these results, we found that, in order to have a non-vanishing lepton asymmetry, some shadow fermions have to decay into lighter messenger scalars, hence could be a candidate "progenitor" for the lepton asymmetry. We argued that this conclusion agrees with unitarity condition of the theory. Then with some further analysis, we ruled out the possibility of having only one messenger scalar field, and found that at least one more heavy messenger scalar is needed for a successful leptogenesis scenario. We then investigated the possibly simplest model and found a rough constraint on the masses of SU(2)z particles. |
