Dark photons from nuclear transitions

Light new particles can be emitted in decays of excited nuclear states. Experiments analyzing such transitions and incorporating high-resolution detectors can be sensitive to new MeV-scale physics at a level competitive with upcoming collider and other fixed target experiments, provided sufficient luminosity. We demonstrate this in the case of the Be-8 system, showing that searches targeting the reported anomaly in Be-8 nuclear transitions can also be sensitive to currently unexplored regions of the canonical dark photon parameter space with 1 MeV less than or similar to m(A') less than or similar to 18 MeV and epsilon(2) greater than or similar to 10(-7). These experiments could be performed on a short time scale, at low cost, and directly probe both the hadronic and leptonic couplings of light hidden particles.