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\ifmmode^\circ\else\textdegree\fi{}, 6.23\ifmmode^\circ\else\textdegree\fi{}, and 7.46\ifmmode^\circ\else\textdegree\fi{}. 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 $R=(\mathrm{experimental}\mathrm{yield})/(\mathrm{theoretical}\mathrm{yield})$ was not 1.0. $R$ was approximately given by $R=0.62{(1.00\ifmmode\pm\else\textpm\fi{}0.05)+\frac{{k}^{2}}{{(4.31\ifmmode\pm\else\textpm\fi{}0.17)}^{2}}}$ where $k$ is the energy in BeV of the photon which produced the pair, and by $R=0.67{(1.00\ifmmode\pm\else\textpm\fi{}0.04)\ensuremath{-}\frac{{{Q}_{f}}^{2}}{{(313\ifmmode\pm\else\textpm\fi{}13)}^{2}}},$ where ${{Q}_{f}}^{2}$ is the four-momentum of the virtual fermion in ${(\mathrm{MeV})}^{2}$. 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.