Shadow fermions, messenger scalars and leptogenesis

Abstract

A mechanism for leptogenesis at the electroweak scale was investigated in a model of dark energy and dark matter proposed by one of us (PQH). 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, namely, 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”. The asymmetry coming from the interference between the tree-level and one-loop amplitudes was calculated for both messenger scalar and shadow fermion decays. It turns out, in order to have a non-vanishing lepton asymmetry and to be consistent with the unitarity condition, some shadow fermions have to decay into lighter messenger scalars, hence could be a candidate “progenitor” for the lepton asymmetry.