New fixed-target experiments to search for dark gauge forces

Fixed-target experiments are ideally suited for discovering new MeV--GeV mass $U(1)$ gauge bosons through their kinetic mixing with the photon. In this paper, we identify the production and decay properties of new light gauge bosons that dictate fixed-target search strategies. We summarize existing limits and suggest five new experimental approaches that we anticipate can cover most of the natural parameter space, using currently operating GeV-energy beams and well-established detection methods. Such experiments are particularly timely in light of recent terrestrial and astrophysical anomalies (PAMELA, Fermi, DAMA/LIBRA, etc.) consistent with dark matter charged under a new gauge force.