Vector-Meson Production by Polarized Photons at 2.8, 4.7, and 9.3 GeV

We present results on vector-meson photoproduction via $\ensuremath{\gamma}p\ensuremath{\rightarrow}\mathrm{Vp}$ in the LBL-SLAC 82-in. hydrogen bubble chamber exposed to a linearly polarized photon beam at 2.8, 4.7, and 9.3 GeV. We find ${\ensuremath{\rho}}^{0}$ production to have the characteristics of a diffractive process, i.e., a cross section decreasing slowly with energy and a differential cross section with slope of \ensuremath{\sim} 6.5 ${\mathrm{GeV}}^{\ensuremath{-}2}$. Within errors the ${\ensuremath{\rho}}^{0}$ production amplitudes are entirely due to natural-parity exchange. $s$-channel helicity is conserved to a high degree in the $\ensuremath{\gamma}\ensuremath{\rightarrow}{\ensuremath{\rho}}^{0}$ transition. We find evidence for small helicity-flip amplitudes for $\ensuremath{\pi}\ensuremath{\pi}$ pairs in the ${\ensuremath{\rho}}^{0}$ region. Photoproduction of $\ensuremath{\omega}$ mesons is separated into its natural- (${\ensuremath{\sigma}}^{N}$) and unnatural- (${\ensuremath{\sigma}}^{U}$) parity-exchange contributions. The ${E}_{\ensuremath{\gamma}}$ and $t$ dependence and the spin density matrix of the unnatural-parity-exchange contribution are consistent with a one-pion-exchange process. The natural-parity-exchange part has characteristics similar to ${\ensuremath{\rho}}^{0}$ production. At 9.3 GeV the ratio of $\ensuremath{\sigma}({\ensuremath{\rho}}^{0})$ to ${\ensuremath{\sigma}}^{N}(\ensuremath{\omega})$ is \ensuremath{\sim} 7. The slope of the $\ensuremath{\varphi}$ differential cross section is \ensuremath{\sim} 4.5 ${\mathrm{GeV}}^{\ensuremath{-}2}$, smaller than that of ${\ensuremath{\rho}}^{0}$ and $\ensuremath{\omega}$ production. Natural-parity exchange is the main contributor to $\ensuremath{\varphi}$ production. No evidence for higher-mass vector mesons is found in $\ensuremath{\pi}\ensuremath{\pi}$, $\ensuremath{\pi}\ensuremath{\pi}\ensuremath{\pi}$, or $K\overline{K}$ final states. The $s$ and $t$ dependences of Compton scattering as calculated from $\ensuremath{\rho}$, $\ensuremath{\omega}$, and $\ensuremath{\varphi}$ photoproduction using vector-meson dominance agree with experiment, but the predicted Compton cross section is too small by a factor of 2.