  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.
