Production of Dark Sector Particles via Resonant Positron Annihilation on Atomic Electrons

Resonant positron annihilation on atomic electrons provides a powerful method to search for light new particles coupled to e^{+}e^{-}. Reliable estimates of production rates require a detailed characterization of electron momentum distributions. We describe a general method that harnesses the target material Compton profile to properly include electron velocity effects in resonant annihilation cross sections. We additionally find that high-Z atoms can efficiently act as particle physics accelerators, providing a density of relativistic electrons that allows one to extend by several times the experimental mass reach.