Search for low-energy resonances in the electron-positron annihilation-in-flight photon spectrum.
We present the results of a search for resonantlike structures in the two-photon decay channel of the ${e}^{+}\ensuremath{-}{e}^{\ensuremath{-}}$ system at energies corresponding to the anomalous peaks seen at Gesellschaft f\"ur Schwerionen-forschung in heavy-ion reactions. Within our sensitivity, no evidence for resonances at the 3% level was found. This enables us to exclude the existence of a Wong-Becker-type particle and to set an upper limit on the ratio $\frac{{\ensuremath{\Gamma}}_{\ensuremath{\gamma}\ensuremath{\gamma}}}{{\ensuremath{\Gamma}}_{{e}^{+}{e}^{\ensuremath{-}}}}\ensuremath{\le}0.7$ at 810 keV.