Testing GeV-Scale Dark Matter with Fixed-Target Missing Momentum
  Experiments
  We describe an approach to detect dark matter and other invisible particles
with mass below a GeV, exploiting missing energy-momentum measurements and
other kinematic features of fixed-target production. In the case of an
invisibly decaying MeV-GeV-scale dark photon, this approach can improve on
present constraints by 2-6 orders of magnitude over the entire mass range,
reaching sensitivity as low as $\epsilon^2\sim 10^{-14}$. Moreover, the
approach can explore essentially all of the viable parameter space for thermal
or asymmetric dark matter annihilating through the vector portal.
