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

Abstract

We investigate phenomenological constraints on a variety of possible explanations of the $e^{+}$ $e^{\mathrm{-}}$ 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→π$e^{+}$ $e^{\mathrm{-}}$ and π→$e^{+}$ $e^{\mathrm{-}}$ $e^{+}$ν, 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 ${10}^{\mathrm{-}14}$–${10}^{\mathrm{-}10}$ sec at high-energy facilities and lifetimes greater than ${10}^{\mathrm{-}10}$ sec at low-energy dumps.