Wide-Angle Electron-Positron Pair Production

Abstract

The yield of wide-angle electron-positron pairs was measured at the Cambridge Electron Accelerator for 7+C -> e + +e~-fC to test the validity of quantum electrodynamics (QED). Symmetric and asymmetric pairs were detected simultaneously, and the interference between the Bethe-Heitler and the Compton matrix elements was measured. Both the angle and the energy of each particle of a pair were measured. One particle was measured with a magnetic spectrometer, using a differential gas Cerenkov counter and shower counters to identify the electron. The other particle of the pair was measured with an array of lead-glass Cerenkov counters. Data were taken at machine energies of 3.0 GeV and 4.14 GeV, and cover an e + e~~ invariant mass range 250-650 MeV/c 2 . This experiment agrees with QED and the results of Asbury et al., and disagrees with the results of Blumenthal et al. The measured interference term is larger than predicted by a model using a p-dominated virtual Compton term.