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

TL;DR

This study presents the first measurement of $J/$ photoproduction off nuclei near threshold energies, revealing insights into gluonic structure and potential modifications in bound protons at high virtuality.

Contribution

It provides the first experimental data on near- and sub-threshold $J/$ photoproduction from nuclei, including cross sections and gluonic radius extraction, and suggests modifications in gluon structure for high-virtuality protons.

Findings

Measured incoherent $J/$ photoproduction cross sections across different nuclei.

Extracted a gluonic radius of bound protons as 0.85 1 0.14 fm.

Observed an excess in sub-threshold production possibly due to modified gluon structure.

Abstract

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.