Photoproduction of Charged Pion Pairs from Hydrogen with Gamma Energies up to 1500 MeV

Charged-pion-pair photoproduction has been investigated up to a $\gamma$ energy of 1500 MeV, using the Caltech 12-in. heavy-liquid bubble chamber with a small-diameter, high-intensity photon beam passing through a central beam-tube gaseous hydrogen target surrounded by the sensitive Freon. Scanning, analysis, and data-reduction techniques have been developed to deal with the problems of two-view stereo, hidden-event origins, absence of magnetic field, and the range-energy and multiple-scattering relationships that occur in the heavy materials. Roughly 5700 pictures have been scanned and analyzed, yielding 754 acceptable events. Cross-section and parameter distributions are generally consistent with the results of previous experiments. The total cross section rises sharply from near zero at 500 MeV to order 80 μb at 650 MeV, then falls slowly to 40 μb at 1500 MeV. The presence of the ${\rho }^0$ is clearly seen in the pion pair production at higher energies.