We present results of measurements on photoproduction of p mesons. Analysis of 10 6 measured p events in a four-dimensional data matrix da(A,m ,p 9 t ± )/d$ldm with dimensions 14x20x10x20 yields precise information on nuclear density distributions for p production. We obtain for the Woods-Saxon radii R{A) = (1.12 ± 0.02)A l/3 and, using the vector dominance model, a N = 26.7 ±2.0 mb and y 2 /47r = 0.57 ±0.10. tion per nucleon |/ 0 | 2 . This yields a pJv and y p 2 / 477 = (a/6477)(a p7V 2 /|/ 0 | 2 ) in a self-consistent manner. Since the original work of Lanzerotti et al., 1 I. several experiments have been done 2,3 to investi-71
Using the DESY 6.2-GeV electron synchrotron, we have measured the photoproduction on carbon of electron-positron pairs, for a range in the invariant pair mass of 150-550 MeV. To detect the pairs and to discriminate against the intense pion background, a magnetic spectrometer, counters, and fast electronics were used. The results are in agreement with the predictions of first-order quantum electrodynamics, in contradiction with earlier experiments. A comparison of our results with other tests of quantum electrodynamics at small distances is discussed.
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