Experimental exclusion of neutral resonances in Bhabha scattering at MeV energies

Abstract

Using an active-shadow technique and a monoenergetic e+ beam at the high-flux reactor of ILL, we pursued the search for long-lived (> 5 × 10-13 s) neutral resonances that could be formed in e+e- scattering around an invariant mass of 1.8 MeV/c2. With a significantly improved experimental sensitivity we found no evidence for such a narrow resonance decaying into e+e- pairs. From our whole investigations, the model-independent stringent lower limits (95% CL) of 5 × 10-11 s and 1 × 10-10 s can be set on the lifetime of hypothetical J = 0 and J = 1 resonances, respectively. Our conclusive results rule out theoretical proposals that involve neutral free particles or e+e- bound states in this mass range, as an explanation of the e+e- lines observed at GSI. © 1991.