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Decisively probing sub-GeV vector-portal dark matter with next generation electron-beam experiments

In a broad class of consistent models, MeV to few-GeV dark matter (DM) interacts with ordinary matter through kinetically-mixed vector mediators ("dark photons"). We outline a two-pronged experimental program to decisively test nearly all such scenarios. The first prong involves placing a suitable meter-scale detector  downstream of an existing electron beam-dump to directly observe DM produced in electron-nucleus collisions. Once produced, the DM  scatters in the detector and induces highly-energetic electron or nuclear recoils. This approach can explore a well-motivated and otherwise inaccessible region of DM parameter space with sensitivity several orders of magnitude beyond existing direct detection and LHC constraints. This approach would also probe invisibly decaying dark-photons down to kinetic mixing of 10 -4 , including the range of parameters relevant for explaining the muon-(g-2) discrepancy. The second, more powerful prong of this discovery program relies entirely on the distinctive kinematics of the DM production in electron-nucleus interactions. In this setup, individual electrons are fired through a thin target adjacent to a tracker and calorimeter. If DM particles are produced as the electron passes through the target, they carry away a large fraction of the incident electron's electron energy. Surprisingly, with suitable trigger and kinematic requirements, such events serve as powerful probes of DM-electron interactions and can explore kinetic mixing parameters down to 10 -7 , which covers nearly all the parameter space consistent with a thermal relic abundance, thereby testing all vector-portal models that have ever achieved thermal equilibrium with the Standard Model.