Wide-Angle Electron-Pair Production

An experiment testing quantum electrodynamics at high energies and small distances is described. The photoproduction from carbon of electron-positron pairs was measured at laboratory angles of 4.60^∘{}, 6.23^∘{}, and 7.46^∘{}. Symmetrical electron-positron pairs in the energy range from 1 to 5 BeV were detected with a magnet-counter system which consisted of two mirror-image arms. Extensive internal checks of the apparatus were made and the results were reproducible. The theoretical values for the electron-pair yield were calculated by integrating the differential pair-production cross section over the acceptance of the apparatus using a Monte Carlo technique. The ratio R=(experimentalyield)/(theoreticalyield) was not 1.0. R was approximately given by R=0.62(1.00±0.05)+k²(4.31±0.17)² where k is the energy in BeV of the photon which produced the pair, and by R=0.67(1.00±0.04)-Qf²(313±13)², where Qf² is the four-momentum of the virtual fermion in (MeV)². The apparatus studies and a comparison of the measured single-electron yields with the theoretical yields suggest that an error exists in the absolute normalization of the results. There are no indications that the observed variation of the electron-pair yields with momentum or the large excess of wide-angle electron pairs at high energies is due to any systematic error. The experimental results do not agree with the predictions of quantum electrodynamics; they indicate a breakdown of the theory or the presence of other processes.