Spin resummation of heavy quarkonium photoproduction: from the gluonic gravitational form factors to the holographic pomeron

Abstract

Exclusive heavy quarkonium photoproduction probes the proton's gluonic structure from near-threshold (fixed-spin exchanges, gravitational form factors) to high energies (reggeized dynamics). We construct a holographic QCD amplitude that resums the even spin-$j$ gluonic exchanges, with the spin-2 input fixed by lattice QCD GFFs. The new framework describes the $J/\psi$ cross section from JLab to HERA energies in a unified manner. It explains why the spin-2 exchange model for GFF extraction near threshold is not a controlled approximation and suggests how to improve it.