Measurement Of Spin-Density Matrix Elements For Phi-Meson Photoproduction From Protons And Deuterons Near Threshold

The LEPS/SPring-8 experiment made a comprehensive measurement of the spin-density matrix elements for gamma p -> phi p, gamma d -> phi pn, and gamma d -> phi d at forward production angles. A linearly polarized photon beam at E-gamma = 1.6-2.4 GeV was used for the production of phi mesons. The natural-parity Pomeron exchange processes remain dominant near threshold. The unnatural-parity processes of pseudoscalar exchange is visible in the production from the nucleons but is greatly reduced in the coherent production from deuterons. There is no strong E-gamma dependence, but there is some dependence on momentum transfer. A small but finite value of the spin-density matrix elements reflecting helicity-nonconserving amplitudes in the t-channel is observed.