First Measurement of Near- and Sub-Threshold \(J/\psi\) Photoproduction off Nuclei

We report on the first measurement of \(J/\psi\) 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 \(\sim8.2\) 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 \(A(\gamma,e^+e^-p)\) reaction with a dilepton invariant mass \(M(e^+e^-)\sim m_{J/\psi}=3.1\) GeV. The incoherent \(J/\psi\) 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 extracting a gluonic radius for bound protons of \(\sqrt{\langle r^2\rangle}=0.85\pm0.14\) fm. The data also suggest an excess of the measured cross section for sub-threshold 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.