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

We report on the measurement of the 𝛾⁢𝑝→𝐽/𝜓⁢𝑝 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 𝑑⁢𝜎/𝑑⁢𝑡 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/𝜓⁢𝑝) and cross sections 𝜎⁡(𝛾⁢𝑝→𝑃+ 𝑐)×ℬ⁡(𝑃+ 𝑐→𝐽/𝜓⁢𝑝).