E+e- peaks at 1.8 MeV: Phenomenological constraints on nuclear transitions, axions, and the particle interpretation

We investigate phenomenological constraints on a variety of possible explanations of the e+e- peaks observed by the EPOS Collaboration at GSI. We first review problems with a variety of proposed nuclear-related production mechanisms. Next we investigate constraints on the axion interpretation of the observed events. New non-accelerator-related constraints come from considerations of shifts in the hyperfine splitting in positronium, and an analysis of the decays K'e+e- and 'e+e-e+, which are relevant for a proposed short-lived axion variant. We then demonstrate that present and past beam-dump experiments are, in a model-independent way, sensitive to a new 1.8-MeV particle coupling to electrons. Our estimates suggest that if the particle does not interact in the target one should see very high event rates for all allowed lifetimes from 10-1410-10 sec at high-energy facilities and lifetimes greater than 10-10 sec at low-energy dumps.