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

• Published in: arXiv.org 2024-10
• Main Authors: Pybus, J R, Ehinger, L, Kolar, T, Devkota, B, Sharp, P, B Yu, Dalton, M M, Dutta, D, Gao, H, Hen, O, Piasetzky, E, Santiesteban, S N, Schmidt, A, Somov, A, Szumila-Vance, H, Adhikari, S, Asaturyan, A, Austregesilo, A, C Ayerbe Gayoso, Barlow, J, Berdnikov, V V, Bhatt, H D, Bhetuwal, Deepak, Black, T, Briscoe, W J, Chung, G, Cole, P L, Deur, A, Dotel, R, Egiyan, H, Eugenio, P, Fanelli, C, Gan, L, Gasparian, A, Guo, J, Hernandez, K, Higinbotham, D W, Hurck, P, Jaegle, I, Jones, R T, Kakoyan, V, Karki, A, H Li, Li, W B, Linera, G R, Lyubovitskij, V, Marukyan, H, McCaughan, M D, McCracken, M, Mizutani, K, Nguyen, D, Oresic, S, Ostrovidov, A I, Papandreou, Z, Paudel, C, Peters, K, Ritman, J, Schick, A, Schwiening, J, Smith, A, Somov, S, Strakovsky, I, Suresh, K, Tarasov, V V, Taylor, S, Xiao, T, Zhang, Z, Zhou, X
• Format: Article
• Language: English
• Subjects: Cross-sections; Deuterium; Momentum transfer; Nuclei; Photon beams; Photons; Photoproduction; Proton beams; Protons
• ISSN: 2331-8422

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.