Photoproduction of Electron Pairs as a Test of Quantum Electrodynamics

As a test of the validity of quantum electrodynamics (QED) for heavy virtual leptons, cross sections for wide-angle electron-pair photoproduction have been measured using hydrogen and carbon targets. Electron pairs were detected in two independent mirror-image spectrometers which identified production angles and momenta. A symmetrical arrangement was used in which interference between Bethe-Heitler and virtual Compton graphs is identically zero. Data were obtained at electron production angles from 5° to 7° and momenta from 1 to 2 GeV/c, and were compared with theory by means of a Monte Carlo calculation. Up to the maximum invariant mass of the electron pair 489 MeV/${\mathit{c}}^2$, the data are consistent with the predictions of QED for both targets, confirming the results of other recent wide-angle pair-production and bremsstrahlung experiments in this energy range. Expressing the results in terms of a cutoff parameter $\Lambda$, where any positive deviation from QED is proportional to ${\Lambda }^{-4\mathrm{}}{Q_l}^4$ (where $Q_l$ is the mass of the virtual lepton), one obtains $\Lambda >0.76\mathrm{}$ GeV/${\mathit{c}}^2$ with 95% confidence. For a similar negative deviation, $\Lambda >0.55\mathrm{}$ GeV/${\mathit{c}}^2$.