Gravitational wave signatures of dark sector portal leptogenesis

We study the possibility of probing leptogenesis via stochastic gravitational waves (GW) arising from a dark sector assisted first-order electroweak phase transition. The same dark sector, with non-trivial transformation under an unbroken $Z_2$ symmetry is also responsible for providing the only source of CP asymmetry via one-loop interference with the tree level decay of a heavy right-handed neutrino into lepton and Higgs doublets. The new Yukawa and scalar portal couplings enhance the CP asymmetry allowing TeV scale leptogenesis without any resonant enhancement. Light neutrino masses arise from a combination of type-I and one-loop contributions with vanishing lightest neutrino mass. While the new degrees of freedom in sub-TeV range keep the detection prospects at terrestrial experiments promising, the new scalars enhance the strength of the electroweak phase transition keeping the GW signals within reach of near future experiments like LISA.