Limits for two-photon and e⁺e^- decay widths of positron-electron scattering resonances for √s=1.78 to 1.92 MeV

Motivated by the observation of energy- and momentum-correlated e⁺e^- pairs in heavy ion collisions, a search for resonances in e⁺e^- scattering at the corresponding energies has been made. The e⁺e^- decay channel is analyzed in a time window from ∼ 10^-13 s to ∼ 10^-11 s with a set-up optimized for low-background detection of delayed e⁺e^- decays. The two-photon decay channel of a hypothetical resonance is investigated by measuring the two-photon annihilation-in-flight excitation function. New upper limits for the partial e⁺e^- decay width Γ_ee of a few meV are derived for total centre-of-mass energies √s between 1.78 and 1.92 MeV, taking into account the dilepton as well as the two-photon decay of a neutral resonance.