Search for Narrow Sum-Energy Lines in Electron-Positron Pair Emission from Heavy-Ion Collisions near the Coulomb Barrier

The first results are presented from a new experiment, APEX, designed to study the previously reported sharp lines in sum-energy spectra of positrons and electrons produced in collisions of very heavy ions. Data were collected for ${}^{238}$U $+$ ${}^{181}$Ta at 5.95, 6.10, and 6.30 MeV/u and ${}^{238}$U $+$ ${}^{232}$Th at 5.95 MeV/u. In none of our analyses is any evidence found for sharp sum-energy lines. For the specific case of the isolated decay of a neutral particle of mass 1.4–2.1 MeV/ $c^2$, the upper limits on cross sections obtained from the present data are significantly less than the previously reported cross sections.