Gluon gravitational form factors of the proton from near-threshold charmonium photoproduction

Abstract

Motivated by near-threshold $J/\psi$ photoproduction measurements, we investigate the gluon contributions to the gravitational form factors (GFFs) of the proton associated with the energy-momentum tensor in quantum chromodynamics. We calculate the gluon GFFs and the corresponding mechanical properties of the proton within a light-front spectator model, in which the proton is described as an active gluon accompanied by a spectator particle. The model parameters are determined from a global fit to the differential cross section for $J/\psi$ photoproduction. The resulting gluon GFFs are in reasonable agreement with recent lattice QCD determinations. We further calculate the total cross section and find reasonable agreement with the available experimental data. Using the extracted gluon GFFs, we investigate the two-dimensional light-front gluon distributions inside the proton, such as the mass, angular momentum, pressure, and shear force distributions. We also study the gluonic contributions to the two-dimensional Galilean energy density, radial and tangential pressures, isotropic pressure, and pressure anisotropy.