Vector meson production by 4.3 GeV polarized photons on deuterium

Abstract

Cross sections for γd and γn interactions and photoproduction of ϱ 0 and ω are studied at 4.3 GeV, using a linearly polarized photon beam in a deuterium bubble chamber. We find that σ T (γ n) ⋍ σ T (γ p) within about 5% and that the γn average charge multiplicity is lower than γp by 0.42±0.09. About 4000 ϱ 0 events and 70 coherent ω events are observed. We present total and differential cross sections for both xoherent and incoherent ϱ 0 production on deuterium, as well as decay angular distributions and density-matrix elements. We find that the t-channel isospin-one exchange amplitude in γN → ϱ 0N (e.g. A 2 exchange) is at most 5–13% of the dominant isoscalar amplituds. The ϱ 0 production mechanism is dominantly s-channel helicity-conserving (SHC) on both neutrons and protons. We find that relative to the SHC amplitudes, the single and double helicity-flip amplitudes at the γϱ 0 vertex are of the order of 10–15% for | t| > 0.25 GeV 2, and have the same sign on both nucleons. This shows that helicity-flip is mainly due to isoscalar exchanges. The ratio of ω to ϱ 0 coherent forward cross sections is found to be 0.16±0.04. The natural-parity exchange part of γN → ωN is strongly dominated by isoscalar exchanges, and the magnitude of the isovector-exchange is consistent with zero.