High-sensitivity measurements of the excitation function for Bhabha scattering at MeV energies

Abstract

Using a monochromatic e+ beam scattered on a Be foil and a high-resolution detector device, the excitation function for elastic e+e- scattering was measured with a statistical accuracy of 0.25% in 1.4-keV steps in the c.m-energy range between 770 and 840 keV (1.79-1.86 MeV/c2) at c.m. scattering angles between 80°and 100°(full width at half maximum). Within the experimental sensitivity of 0.5 b eV/sr (c.m.) for the energy-integrated differential cross section no resonances were observed (97% C.L.). From this limit we infer that a hypothetical spinless resonant state should have a width of less than 1.9 meV corresponding to a lifetime limit of 3.5×10-13 s. This limit establishes the most stringent bound for new particles in this mass range derived from Bhabha scattering and is independent of assumptions about the internal structure of the hypothetical particles. Less sensitive limits were, in addition, derived around 520-keV c.m. energy (1.54 MeV/c2) from an investigation with a thorium and a Mylar foil as scatterers. © 1989 The American Physical Society.