Shadow fermions, messenger scalars and leptogenesis Original Research Article

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 image and an axion-like particle with an image-instanton-induced potential which yields a scenario for the dark energy. Furthermore, the extended particle content of this image model contains a possible candidate for the cold dark matter, namely, the image “shadow” fermion, which couples with the Standard Model lepton through a scalar “messenger field” carrying both image and electroweak quantum numbers. Since these shadow fermions are in a real representation of image, lepton number can be violated in the Yukawa sector and a lepton number asymmetry can be generated in the image particleʹs 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.