Electroweak baryogenesis from a dark sector
  Adding an extra singlet scalar $S$ to the Higgs sector can provide a barrier
at tree level between a false vacuum with restored electroweak symmetry and the
true one. This has been demonstrated to readily give a strong phase transition
as required for electroweak baryogenesis. We show that with the addition of a
fermionic dark matter particle $\chi$ coupling to $S$, a simple UV-complete
model can realize successful electroweak baryogenesis. The dark matter gets a
CP asymmetry that is transferred to the standard model through a $CP\ portal\
interaction$, which we take to be a coupling of $\chi$ to $\tau$ leptons and an
inert Higgs doublet. The CP asymmetry induced in left-handed $\tau$ leptons
biases sphalerons to produce the baryon asymmetry. The model has promising
discovery potential at the LHC, while robustly providing a large enough baryon
asymmetry and correct dark matter relic density with reasonable values of the
couplings.
