First Measurement of Near-Threshold J/ψ Exclusive Photoproduction off the Proton

We report on the measurement of the gamma p -> J/psi p cross section from E-gamma = 11.8 GeV down to the threshold at 8.2 GeV using a tagged photon beam with the GlueX experiment. We find that the total cross section falls toward the threshold less steeply than expected from two-gluon exchange models. The differential cross section d sigma/dt has an exponential slope of 1.67 +/- 0.39 GeV-2 at 10.7 GeV average energy. The LHCb pentaquark candidates P-c(+) can be produced in the s channel of this reaction. We see no evidence for them and set model-dependent upper limits on their branching fractions B(P-c(+) -> J/psi p) and cross sections sigma(gamma p -> P-c(+)) x B(P-c(+) -> J/psi p).