Abstract
The differential cross sections and decay angular distributions for coherent ϕ-meson photoproduction from helium-4 are measured for the first time at forward angles with linearly polarized photons in the energy range Eγ=1.685–2.385GeV. Thanks to the target with spin-parity JP=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 (θ=0∘) for the γp→ϕp reaction near threshold, the energy dependence of the forward cross section (θ=0∘) for the γHe4→ϕHe4 reaction is analyzed. The comparison to γp→ϕp 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 γp→ϕp reaction near threshold.Received 3 November 2017Revised 21 January 2018DOI:https://doi.org/10.1103/PhysRevC.97.035208©2018 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasParticle interactionsParticle productionPhotonuclear reactionsParticles & FieldsNuclear Physics
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