Scattering of 150- and 300-MeV Positrons and Electrons from Bismuth

Positrons and electrons from the Orsay linear accelerator were scattered from bismuth at 6°, 10°, 15°, and 20° at 300 MeV, and at angles having similar momentum transfers at 150 MeV. In addition to electron-positron ratios, relative and absolute cross sections were measured and are given here for the present experiment as well as for the high- and the low-resolution experiments reported earlier. The latter data extend to 45° at 300 MeV ($q^2\approx 1.4 {\mathrm{F}}^{-2\mathrm{}}$). The combined data from the three experiments are internally consistent and are in general agreement with the partial-wave predictions based on earlier electron scattering results. However, the electron-positron ratio in the neighborhood of the 6°, 300-MeV point disagrees by 2.2 standard deviations with the partial-wave predictions and also with the value for the ratio obtained at the corresponding 150-MeV point when the latter is scaled to 300 MeV by means of a "model-independent" scaling law. The present data thus confirm the discrepancy noted earlier in which the results of Goldemberg, Pine, and Yount at 5° and 300 MeV disagreed with the partial-wave predictions and with the scaled results of Miller and Robinson taken at energies below 170 MeV.