Unitarized Matching of Gluon PDF Moments and the QCD Trace Anomaly in Near-Threshold $J/\psi$ Photoproduction

TL;DR

This paper develops a unitarized theoretical framework for near-threshold $J/ar{psi}$ photoproduction, combining gluon PDF moments and QCD trace anomaly effects, and validates it against experimental data.

Contribution

It introduces a novel unitarized model that incorporates target-mass corrections and scalar form factors, unifying threshold and high-energy regimes in $J/ar{psi}$ photoproduction.

Findings

The model fits GlueX data well and reveals signatures in cross sections and $t$-slopes.

Scalar form-factor effects are robust against PDF variations.

A smooth transition to Pomeron behavior at high energies is achieved.

Abstract

We construct a framework for near-threshold $J/\psi$ photoproduction that combines a target-mass-corrected gluon-PDF moment expansion with an explicit scalar gravitational form factor associated with the QCD trace anomaly. Both contributions are embedded into a common partial-wave K-matrix unitarization scheme, ensuring elastic unitarity at the amplitude level. The resulting hierarchy of nested models is tested against GlueX data and shown to produce distinct signatures in the differential cross section, the effective $t$-slope, and the real-to-imaginary ratio. The scalar form-factor signal is found to be robust against PDF-set variations and survives unitarization. A smooth matching to the Pomeron power-law regime at high energy is demonstrated, providing a unified description from threshold to HERA energies spanning three orders of magnitude in the center-of-mass energy.