Near-threshold photon-proton production of $J/\psi$ and ${\rm Y}$

Abstract

We study the near-threshold exclusive photoproduction of heavy vector mesons (quarkonia $J/\psi$ and ${\rm Y}$) 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 ${\rm Y}$ photoproduction, which can be tested at future Electron-Ion Colliders.

Comments: 16 pages, 9 figures.typo corrected