Study of High-Energy Photoproduction with Positron-Annihilation Radiation. I. Three-Prong Events

The photoproduction of resonances has been studied in three exposures of a hydrogen bubble chamber to positron-annihilation radiation of 4.3-, 5.25-, and 7.5-GeV nominal energies. The general analysis procedure and results on the three-prong-event topology are presented. We study the highly constrained reaction $\gamma p\to p{\pi }^{+}{\pi }^{-}$ over the energy range 2-8 GeV and the reactions $\gamma p\to p{\pi }^{+}{\pi }^{-}{\pi }^0$ and $\gamma p\to n{\pi }^{+}{\pi }^{+}{\pi }^{-}$ at the annihilation energies. Cross sections are given. Using various models to parametrize the ${\rho }^0$ "elastic" reaction we find the mean $t$ slope to be 7.1±0.4 Ge${\mathrm{V}}^{-2\mathrm{}}$ and its forward cross section to decrease from 130 μb Ge${\mathrm{V}}^{-2\mathrm{}}$ at 3.3 GeV to ∼ 100 μb Ge${\mathrm{V}}^{-2\mathrm{}}$ at 7.5 GeV. The cross section for $\gamma p\to \omega p$ is decomposed into one-pion exchange (OPE) and a diffractive part, ${\sigma }_D\left(\omega \right)$, which is found to be 1.5±0.3 μb. $\frac{\sigma \left({\rho }^0\right)}{{\sigma }_D\left(\omega \right)}$ is then 9.5±2.3. In comparing vector-meson production with Compton scattering via vector-meson dominance, we find that for $\frac{{{\gamma }_{\rho }}^2}{4\pi }=0.32\mathrm{}\pm 0.03$ there is good agreement at all $s$ and $t$ where comparison may be made ($E_{\gamma }=5-8\mathrm{}$ GeV). In a search for ${\rho }^{\prime }\to 2\pi$, we find, at the 90% confidence level, ${\sigma }_{{\rho }^{\prime }}\mathrm{}\left(1250\right)<0.3\mathrm{}$ μb and ${\sigma }_{{\rho }^{\prime }}\mathrm{}\left(1650\right)<0.1\mathrm{}$ μb per 100-MeV width. Inelastic peripheral ${\rho }^0$ production is seen, but with present statistics we cannot identify specific nucleon isobars associated with it. Quasi-two-body reactions $\gamma p\to {\pi }^{-}{\Delta }^{++}$, ${\rho }^{-}{\Delta }^{++}$, and $A_2^{+}n$ are observed, decreasing with photon energy like ${E_{\gamma }}^{-a}$. For the first two we find $a=1.74\mathrm{}\pm 0.16$ and 0.6±0.2, respectively. We conclude that if the reaction $\gamma p\to {\rho }^{-}{\Delta }^{++}$ is due to an OPE process, the required $\rho \to \pi \gamma$ width (∼0.5 MeV) is much in excess of the value predicted by SU(3).