Kinetic Mixing from Kaluza-Klein Modes: A Simple Construction for Portal Matter

Abstract

The vector portal/kinetic mixing simplified model of dark matter, in which thermal dark matter of a mass ranging from a few MeV to a few GeV can be realized with a dark-sector $U(1)$, relies on a small kinetic mixing term between this dark $U(1)$ and the Standard Model (SM) hypercharge. It is well-known that kinetic mixing of the right magnitude can be generated at one loop by the inclusion of "portal matter" fields which are charged under both the dark $U(1)$ and the SM hypercharge, and it has been previously argued on phenomenological grounds that fermionic portal matter fields must exhibit a specific set of characteristics: They must be vector-like and have the same Standard Model group representations as existing SM fermions. A natural explanation for the presence of dark $U(1)$-charged copies of SM fermions would be to enlarge the dark gauge group and embed the SM and the portal matter into a single dark multiplet, however, previous works in this direction require significant ad hoc additions to ensure that the portal matter fields are vector-like while the SM fields are chiral, and rely on complicated dark Higgs sectors to realize the appropriate symmetry breaking. In this paper, we argue that a model with large extra dimensions can easily generate chiral SM fermions and vector-like portal matter from a single dark bulk multiplet, while allowing for a far simpler dark Higgs sector. We present a minimal construction of this "Kaluza-Klein (KK) portal matter" and explore its phenomenology at the LHC, noting that even our simple realization exhibits signatures unique from those of more conventional models with large extra dimensions and 4-D portal matter. Generically, the portal matter sector fields will be much lighter than the other KK fields in the model, so the portal matter sector has the potential to be the first observable experimental signature of an extra dimension.