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

We present results on vector-meson photoproduction via $\gamma p\to \mathrm{Vp}$ 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 ${\rho }^0$ 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 ∼ 6.5 ${\mathrm{GeV}}^{-2\mathrm{}}$. Within errors the ${\rho }^0$ production amplitudes are entirely due to natural-parity exchange. $s$-channel helicity is conserved to a high degree in the $\gamma \to {\rho }^0$ transition. We find evidence for small helicity-flip amplitudes for $\pi \pi$ pairs in the ${\rho }^0$ region. Photoproduction of $\omega$ mesons is separated into its natural- (${\sigma }^N$) and unnatural- (${\sigma }^U$) parity-exchange contributions. The $E_{\gamma }$ 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 ${\rho }^0$ production. At 9.3 GeV the ratio of $\sigma \left({\rho }^0\right)$ to ${\sigma }^N\left(\omega \right)$ is ∼ 7. The slope of the $\phi$ differential cross section is ∼ 4.5 ${\mathrm{GeV}}^{-2\mathrm{}}$, smaller than that of ${\rho }^0$ and $\omega$ production. Natural-parity exchange is the main contributor to $\phi$ production. No evidence for higher-mass vector mesons is found in $\pi \pi$, $\pi \pi \pi$, or $K\overline{K}$ final states. The $s$ and $t$ dependences of Compton scattering as calculated from $\rho$, $\omega$, and $\phi$ photoproduction using vector-meson dominance agree with experiment, but the predicted Compton cross section is too small by a factor of 2.