Search for resonances in electron-positron scattering using a cold positron gas target

Summary form only given. The recently discovered correlated positron-electron peaks emitted from superheavy nuclear collisions may be signatures for previously undetected neutral particlelike objects having masses of 1-2 MeV/c². An experiment has been designed to definitively test this hypothesis by searching for resonant states formed directly in the scattering of monoenergetic electrons incident on a gas of cold positrons confined in a Malmberg-type Penning trap. This technique will provide an improvement in sensitivity for detecting new neutral particlelike objects of two orders of magnitude over that of present positron-beam, thin-foil scattering experiments, which are fundamentally limited by the momentum of the atomic electrons. Combined with a recoil-shadow technique, this experiment will explore the entire four-decade range of possible neutral particle lifetimes (10^-13 , s to 10^-9 s), which cannot be probed by other methods. Construction is underway. Positrons will be produced at the Lawrence Livermore National Laboratory 100-MeV Linac and trapped in a 6-T solenoid after remoderation. A 3-MeV pelletron will be used as a variable energy electron source. The scattered particles will be analyzed by a transverse magnetic field downstream. The electron, positron, and annihilation photons will be detected in coincidence. Large angle scattering events are anticipated at a 0.1-1.0-Hz rate