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 ε²~10^-14. Moreover, the approach can explore essentially all of the viable parameter space for thermal or asymmetric dark matter annihilating through the vector portal.