First measurement of coherent phi-meson photoproduction from He-4 near threshold

Abstract

The differential cross sections and decay angular distributions for coherent phi-meson photoproduction from helium-4 are measured for the first time at forward angles with linearly polarized photons in the energy range E-gamma = 1.685-2.385 GeV. Thanks to the target with spin-parity J(P) = 0(+), unnatural-parity exchanges are absent, and thus natural-parity exchanges can be investigated clearly. The decay asymmetry with respect to photon polarization is shown to be very close to the maximal value. This ensures the strong dominance (> 94%) of natural-parity exchanges in this reaction. To evaluate the contribution from natural-parity exchanges to the forward cross section (theta = 0 degrees) for the gamma p -> phi p reaction near threshold, the energy dependence of the forward cross section (theta = 0 degrees) for the gamma He-4 -> phi He-4 reaction is analyzed. The comparison to gamma p ->phi p data suggests that enhancement of the forward cross section arising from natural-parity exchanges and/or destructive interference between natural-parity and unnatural-parity exchanges is needed in the gamma p ->phi p reaction near threshold.