Secluded WIMPs, QED with massive photons, and the galactic center
  gamma-ray excess
  We propose to investigate a secluded WIMP dark matter model consisting of
neutral fermions as the dark matter candidate and a Proca-Wentzel (PW) field as
a mediator. In the model that we consider here, dark matter WIMPs interact with
standard model (SM) particles only through the PW field of ~ MeV -- multi-GeV
mass particles. The interactions occur via an U(1)' mediator, V_{\mu}', which
couples to the SM by kinetic mixing with U(1) hypercharge bosons, B_{\mu}. One
important difference between our model and other such models in the literature
is the absence of an extra singlet scalar, so that the parameter with dimension
of mass M^2_V is not related to a spontaneous symmetry breaking. This QED based
model is also renormalizable. The mass scale of the mediator and the absence of
the singlet scalar can lead to interesting astrophysical signatures. The
dominant annihilation channels are different from those usually considered in
previous work. We show that the GeV-energy gamma-ray excess in the galactic
center region, as derived from Fermi-LAT Gamma-ray Space Telescope data, can be
attributed to such secluded dark matter WIMPs, given parameters of the model
that are consistent with the cosmological dark matter density. Secluded WIMP
models are also consistent with suggested upper limits on the DM contribution
to the cosmic-ray antiproton flux.
