Near-threshold photon-proton production of J / ψ and ϒ

We study the near-threshold exclusive photoproduction of heavy vector mesons (quarkonia J / ψ and ϒ ) 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 from global analyses. We compare our results with the latest near-threshold J / ψ production data from the GlueX and J / ψ − 007 experiments at Jefferson Laboratory, finding good agreement for both differential and total cross sections. Predictions are also provided for ϒ photoproduction, which can be tested at future electron-ion colliders.