Near-threshold photon-proton production of $J/\psi$ and $\Upsilon$

We study the near-threshold exclusive photoproduction of heavy vector mesons (quarkonia $J/\psi$ and $\Upsilon$) off the proton within the framework of generalized parton distribution (GPD) factorization. The gluon GPDs are computed using a spectator model in which the proton emits a gluon and the remaining constituents are treated as a single spectator particle. Model parameters are determined by fitting the gluon unpolarized and helicity collinear parton distribution functions (PDFs) from global analyses. We compare our results with the latest near-threshold $J/\psi$ production data from the GlueX and $J/\psi$-007 experiments at Jefferson Laboratory, finding good agreement for both differential and total cross sections. Predictions are also provided for $\Upsilon$ photoproduction, which can be tested at future Electron-Ion Colliders.