Abstract
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.
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