First Measurement of Near- and Sub-Threshold J/ψ Photoproduction off Nuclei First Measurement of Near- and Sub-Threshold J/ψ Photoproduction off Nuclei

We report on the first measurement of J/ψ photoproduction from nuclei in the photon energy range of 7 to 10.8 GeV, extending above and below the photoproduction threshold in the free proton of ∼8.2  GeV. The experiment used a tagged photon beam incident on deuterium, helium, and carbon, and the GlueX detector at Jefferson Lab to measure the semi-inclusive A(γ,e+e-p) reaction with a dilepton invariant mass M(e+e-)∼mJ/ψ=3.1  GeV. The incoherent J/ψ photoproduction cross sections in the measured nuclei are extracted as a function of the incident photon energy, momentum transfer, and proton-reconstructed missing light-cone momentum fraction. Comparisons with theoretical predictions assuming a dipole form factor allow for extracting a gluonic radius for bound protons of ⟨r2⟩=0.85±0.14  fm. The data also suggest an excess of the measured cross section for subthreshold production and for interactions with high missing light-cone momentum fraction protons. The measured enhancement can be explained by modified gluon structure for high-virtuality bound protons.