Search for a Bhabha-scattering resonance near 1.8 MeV/c2

Abstract

We use a magnetic spectrometer with a Si(Li) detector to observe the Bhabha scattering of collimated ${}_{}{}^{82}{}_{}{}^{}\mathrm{-}_{}^{82}{}_{}{}^{}$Rb β-decay positrons in a thin Be foil. We find no evidence for any $e^{+}$-$e^{\mathrm{-}}$ resonances in the vicinity of the 1.8-MeV/$c^2$ mass suggested by the $e^{+}$-$e^{\mathrm{-}}$ events observed in heavy-ion collisions near the Coulomb barrier. At 1.8 MeV, the 90%-confidence upper limit on the product of the resonance cross section and its width is 1 b keV for widths less than 6 keV. We infer that a singlet resonance coupling only to $e^{+}$-$e^{\mathrm{-}}$ must have a width narrower than 2.5 eV.