Threshold Photoproduction of J/ψ at GlueX*

The GlueX experiment at Jefferson Lab uses a 12 GeV electron beam to produce tagged photons via coherent Bremsstrahlung. This linearly-polarized photon beam impinges on a liquid-hydrogen target contained within a hermetic detector with near-complete coverage by drift chambers and electromagnetic calorimeters. The available energy range is ideal to study J/ψ production close to threshold at 8.2 GeV, and the energy-dependence of the cross-section is very sensitive to the details of the production mechanism. The cross-section is also closely related to the s-channel production of the pentaquark resonances P_c⁺(4380) and P_c⁺(4450) observed by LHCb in J/ψp systems. Taking advantage of the exclusivity of the reaction and the precise knowledge of the beam energy, the GlueX experiment achieves a fine mass resolution and a pure data sample. We will present preliminary results for the cross-section and an upper limit on the P_c⁺ → J/ψp branching fraction.