QCD Analysis of Near-Threshold Photon-Proton Production of Heavy Quarkonium

TL;DR

This paper investigates near-threshold heavy quarkonium production off protons using QCD, highlighting its potential to probe gluonic structure and form factors of the proton through recent and future experimental data.

Contribution

It introduces a QCD framework for analyzing near-threshold quarkonium production, emphasizing the extraction of gluonic gravitational form factors from threshold data.

Findings

Threshold production is dominated by spin-2 contributions.

Data can reveal gluonic contributions to proton's mass and pressure.

Future experiments will enhance understanding of proton structure.

Abstract

The near threshold photo or electroproduction of heavy vector quarkonium off the proton is studied in quantum chromodynamics. Similar to the high-energy limit, the production amplitude can be factorized in terms of gluonic Generalized Parton Distributions and the quarkonium distribution amplitude. At the threshold, the threshold kinematics has a large skewness parameter $\xi$, leading to the dominance of the spin-2 contribution over higher-spin twist-2 operators. Thus threshold production data are useful to extract the gluonic gravitational form factors, allowing studying the gluonic contributions to the quantum anomalous energy, mass radius, spin and mechanical pressure in the proton. We use the recent GlueX data on the $J/\psi$ photoproduction to illustrate the potential physics impact from the high-precision data from future JLab 12 GeV and EIC physics program.