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.

• Received 8 January 2015

DOI:https://doi.org/10.1103/PhysRevD.91.094026