Towards a UV model of kinetic mixing and portal matter

The nature of dark matter (DM) and how it might interact with the particles of the Standard Model (SM) is an ever-growing mystery. It is possible that the existence of new "dark sector" forces, yet undiscovered, are the key to solving this fundamental problem, and one might hope that in the future such forces might even be "unified" with the ones we already know in some UV-complete framework. In this paper, following a bottom-up approach, we attempt to take the first steps in the construction of such a framework. The much-discussed possibility of the kinetic mixing (KM) of the "dark photon" with the hypercharge gauge boson of the SM via loops of portal matter (PM) fields, charged in both sectors, offers an attractive starting point for these efforts. Given the anticipated finite strength of the KM in a UV-complete theory, the absence of anomalies, and the lifetime constraints on the PM fields arising from cosmic microwave background and nucleosynthesis constraints, PM must behave as vector-like copies of the known SM fermion fields, such as those which appear naturally in, e.g., E-6-type models. Within such a setup, the SM and their corresponding partner PM fields would be related by a new SU(2)(1) gauge symmetry. With this observation as a springboard, we construct a generalization of these ideas where SU(2), is augmented by an additional U(1)(1Y) factor so that the light dark photon is the result of a symmetry breaking analogous to the SM, i.e., SU(2)(1) x U(1)(1Y) -> U(1)(D), but with U(1)(D) now also broken at the