Vector-meson production by polarized photons at 2.8, 4.7, and 9.3 GeV

We present results on vector-meson photoproduction via γ p→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 ρ⁰ 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 GeV⁻². Within errors the ρ⁰ production amplitudes are entirely due to natural-parity exchange. s-channel helicity is conserved to a high degree in the γ→ρ⁰ transition. We find evidence for small helicity-flip amplitudes for ππ pairs in the ρ⁰ region. Photoproduction of ω 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 ρ⁰ production. At 9.3 GeV the ratio of σ(ρ⁰) to ${\sigma }^N\left(\omega \right)$ is ∼ 7. The slope of the φ differential cross section is ∼ 4.5 GeV⁻², smaller than that of ρ⁰ and ω production. Natural-parity exchange is the main contributor to φ production. No evidence for higher-mass vector mesons is found in ππ, πππ, or KK̄ final states. The s and t dependences of Compton scattering as calculated from ρ, ω, and φ photoproduction using vector-meson dominance agree with experiment, but the predicted Compton cross section is too small by a factor of 2.