Leptogenesis in a model of Dark Energy and Dark Matter

A recent model of dark energy and dark matter was proposed, involving a new gauge group $SU(2)_Z$ whose coupling grows strong at a scale $\Lambda_Z \sim 10^{-3} eV$, a result which is obtained from a simple assumption that its initial value at some high energy scale $M \sim 10^{16} GeV$ is of the order of a typical Standard Model (SM) coupling at a similar scale. (This assumption comes naturally from an embedding of $SU(2)_Z$ and the SM into a grand unified group $E_6$.) It is found that the proposed model contains a SM lepton-number violating Yukawa coupling involving a scalar ``messenger field'' ${\tilde{\bm{\phi}}}^{(Z)}$ (which carries both $SU(2)_Z$ and electroweak quantum numbers), a $SU(2)_Z$ fermion $\psi^{(Z)}$ and a SM lepton $l$. The interference between the tree-level and one-loop decay amplitude for ${\tilde{\bm{\phi}}}^{(Z)} \to \psi^{(Z)} + l$ generates a SM lepton asymmetry which is subsequently converted into a baryon asymmetry through electroweak sphaleron processes. This constrains the mass of the messenger field to be less than $1 TeV$, making it accessible to searches at future colliders: the ``progenitor'' of a net lepton number (and hence a net baryon number) could possibly be found and identified experimentally.