π-Pair Photoproduction below 1 BeV

Measurements of the differential cross section for the reaction $\gamma +p\to {\pi }^{-}+{\pi }^{+}+p$ have been made at several angles for photon energies in the range 550-1000 MeV, using the Stanford Mark III linear accelerator. The ${\pi }^{-}$ were detected and momentum-analyzed using a 90° magnetic spectrometer. It was found that the reaction was dominated by the quasi-two-body photoproduction $\gamma +p\to {\pi }^{-}+N^{*}$ (1238) especially near the threshold for this process. At low momenta, the ${\pi }^{-}$ were identified by range. At high momenta (>250 $\frac{\mathrm{MeV}}{c}$), the contaminating electrons were eliminated by using a lead and scintillation-counter sandwich system. The yields of both ${\pi }^{-}$ and ${\pi }^{+}$ from hydrogen were measured, and the normalization was obtained by comparison with the known cross sections for $\gamma +p\to {\pi }^{+}+n$. Computer calculations of the shapes of the yield curves expected from two-body and three-body production enabled the data to be separated into the two possible states $\gamma +p\to {\pi }^{-}+N^{*}$ (1238) and $\gamma +p\to {\pi }^{-}+{\pi }^{+}+p$. Angular distributions and total cross sections are presented.