Search for resonant Bhabha scattering at MeV energies

Abstract

A series of investigations on Bhabha scattering has been carried out in the last two years, searching for possible neutral resonances in the invariant mass range between 1.5 and 1.9 MeV/c2, as motivated by the heavy-ion experiments at GSI. The currently highest sensitivity has been achieved by an experiment performed at ILL using the BILL β spectrometer to produce monoenergetic positrons and a high-resolution multidetector device. With systematic and statistical uncertainties of less than 0.25%, no evidence for any resonance was found around 1.8 MeV/c2. The 97%-confidence upper limit on the intrinsic width and lifetime of a hypothetical spinless resonance is 1.9 meV and 3.5 × 10-13 s, respectively, establishing the most stringent bound for new particles in this mass region. © 1989.