First Measurement of Near-Threshold $J/\psi$ Exclusive Photoproduction off the Proton

Abstract

We report on the measurement of the γp → J/ψp cross section from Eγ = 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 dσ / dt has an exponential slope of 1.67 ± 0.39 GeV−2. at 10.7 GeV average energy. The LHCb pentaquark candidates Pc+ 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(Pc+ → J/ψp) and cross sections σ(γp → Pc+) x B(Pc+ → J/ψp).