Evidence for a resonance in positron-electron scattering at 810 keV centre-of-mass Energy

A monoenergetic positron (e ⁺) beam (ΔT/T<5·10^?4) from the Stuttgart pelletron accelerator and a 4.6 mg/cm² Be target has been employed to investigatee ⁺ e ^?scattering in the MeV region in a large solid angle with high statistics by means of a novel positionsensitive detector system. Superimposed on the Bhabha scattering, a structure has been found at ane ⁺ kinetic energy of 2263 keV (810 keV excitation energy in thee ⁺ e ^?rest frame). From the energy-integrated resonance cross-section of \(\sigma _{res} \cdot \Gamma _{res}^{e^ + e^ - } \) ?30 b·eV (c.m.) and the standard resonance cross-section we estimate partial resonance widths \(\Gamma _{res}^{e^ + e^ - } \) ?72 meV or 24 meV for total angular momenta J=0 or J=1, respectively. The structure, which has not been predicted within the framework of quantum electrodynamics, coincides with one of the sum energies of the correlatede ⁺ e ^?lines observed in heavy-ion collisions.