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

TL;DR

This work applies GPD factorization to near-threshold photoproduction of heavy quarkonia (Jpsi and Upsilon) to access large-ξ gluon structure in the proton. Using a proton spectator model, it computes twist-2 gluon GPDs and fixes model parameters by fitting unpolarized and helicity gluon PDFs from NNDPF, obtaining moments that agree with lattice results for gluonic contributions to the proton energy–momentum tensor. The study reproduces near-threshold J/psi data from GlueX and JLab and delivers predictions for Upsilon photoproduction suitable for future Electron–Ion Colliders, highlighting the connection between cross sections and gluon gravitational form factors. Overall, the approach provides a coherent framework to extract 3D gluon structure at large skewness and to test aspects of the gluonic sector in QCD.

Abstract

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 (PDFs) 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.

Figures (9)

• Figure 1: Examples of leading Feynman diagrams that contribute to the exclusive photoproduction of the heavy vector meson.
• Figure 2: Kinematics for the heavy vector-meson photoproduction in the center-of-mass frame.
• Figure 3: Left panel: $\xi$ values on the $(W,-t)$ plane in the kinematically allowed region with $M_{J/\psi}=3.097\,\text{GeV}$. Right panel: $\xi$ values on the $(W,-t)$ plane in the kinematically allowed region with $M_{\Upsilon}=9.46\,\text{GeV}$.
• Figure 4: Kinematics for gluon GPDs in the spectator model. The gluon lines with crosses correspond to the specific Feynman rules for the gluon field strength tensor.
• Figure 5: The simultaneous fit of the model results in Eqs. (\ref{['eq:f1g']}-\ref{['eq:g1g']}) to the NNPDF3.1 NLO dataset for $xf_1^g(x)$NNPDF:2017mvq (left panel) and the NNPDFpol1.1 NLO dataset for $xg_{1L}^g(x)$Nocera:2014gqa (right panel).