$Z'$ Mediated WIMPs: Dead, Dying, or Soon to be Detected?
  Although weakly interacting massive particles (WIMPs) have long been among
the most studied and theoretically attractive classes of candidates for the
dark matter of our universe, the lack of their detection in direct detection
and collider experiments has begun to dampen enthusiasm for this paradigm. In
this study, we set out to appraise the status of the WIMP paradigm, focusing on
the case of dark matter candidates that interact with the Standard Model
through a new gauge boson. After considering a wide range of $Z'$ mediated dark
matter models, we quantitatively evaluate the fraction of the parameter space
that has been excluded by existing experiments, and that is projected to fall
within the reach of future direct detection experiments. Despite the existence
of stringent constraints, we find that a sizable fraction of this parameter
space remains viable. More specifically, if the dark matter is a Majorana
fermion, we find that an order one fraction of the parameter space is in many
cases untested by current experiments. Future direct detection experiments with
sensitivity near the irreducible neutrino floor will be able to test a
significant fraction of the currently viable parameter space, providing
considerable motivation for the next generation of direct detection
experiments.
