ABSTRACT:
In a broad class of consistent models, MeV to few-GeV dark matter interacts with ordinary matter through weakly coupled GeV-scale mediators. A suitable meter-scale (or smaller) detector situated downstream of an existing electron beam-dump can sensitively probe dark matter interacting via sub-GeV mediators. These experiments can explore a well-motivated and otherwise inaccessible region of dark matter parameter space with sensitivity several orders of magnitude beyond existing direct detection constraints. This approach would also probe invisibly decaying new MeV-GeV gauge bosons ("dark photons") down to kinetic mixing of \epsilon ~ 10^{-4}, including the range of parameters relevant for explaining the (g-2)_{\mu} discrepancy. Sensitivity to other long-lived dark sector states and to new milli-charge particles would also be improved.