The reaction γ + p p + π+ + π- has been studied for photon energies between 800 and 1500 MeV and for dipion masses between 510 and 900 MeV. The bremsstrahlung beam from the Caltech synchrotron was passed through a liquid hydrogen target and spark chambers were used to detect the three final particles. In addition, the proton energy was determined by a range measurement. Approximately 40,000 photographs were taken, yielding 3018 acceptable events. The results were fit to an incoherent combination of the N*(1238) resonance, the po (750) resonance, and three-body phase space, with various models being tried for poproduction. The total cross section for po production is consistent with previous experiments. However, the angular dependence of the cross section is slightly more peaked in the forward direction, and the ratio of po production to phase space production is larger than previously observed.
However, since this experiment was only sensitive to the production angles cos θ cm ≥ .75, statistical fluctuations and/or an anisotropic distribution of background production have a severe influence on the po to background ratio. Of the po models tested, the results prefer po production by the one pion exchange mechanism with a very steep form factor dependence. The values of the mass and width of the po found here are consistent with previous experiments.
