Wide-Angle Electron-Positron Pair Production

Abstract

The yield of wide-angle electron-positron pairs was measured at the Cambridge Electron Accelerator for $\gamma +\mathrm{C}\to e^{+}+e^{-}+\mathrm{C}$ 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 Čerenkov counter and shower counters to identify the electron. The other particle of the pair was measured with an array of lead-glass Čerenkov 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 $\frac{\mathrm{MeV}}{c^2}$. 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 $\rho$-dominated virtual Compton term.