Near-Threshold J/$\psi \to \mu^+\mu^-$ Photoproduction and the Gluonic Gravitational Form Factors of the Proton

TL;DR

This study measures near-threshold J/psi photoproduction to probe the gluonic gravitational form factors of the proton, providing experimental constraints consistent with lattice QCD and supporting a gluon-dominated spatial structure.

Contribution

It presents the first measurement of the near-threshold J/psi photoproduction cross section in muon decay channel and constrains the gluonic gravitational form factor using a combined analysis with holographic QCD.

Findings

Results agree with previous electron decay channel measurements.

No evidence found for open-charm contributions.

Constraints on gluonic gravitational form factor align with lattice QCD calculations.

Abstract

We report on the measurement of the two-dimensional differential cross section for near-threshold J/$\psi \to \mu^+\mu^-$ photoproduction from the J/$\psi$-007 experiment in Hall C at Jefferson Lab. Our results agree with the previously published J/$\psi \to e^+e^-$ results. We extract the integrated photoproduction cross section and find no evidence for open-charm contributions. A combined analysis of both decay channels following a Holographic QCD approach yields improved experimental constraints on the gluonic gravitational form factor $\mathcal{C}_g(t)$. Our results agree with recent lattice QCD calculations and we obtain $\mathcal{C}_g(t)$ with a comparable statistical precision to lattice QCD. Our results support a spatial picture where gluons dominate at larger radii with a confining inward pressure. This work provides new input for exploring the mechanical properties of gluons inside the proton.