  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.
