Gluon gravitational form factors and the proton mass radius
In plain words
How mass and pressure are spread inside the proton can be read from its response to gravity-like probes, approximated in practice by producing heavy $J/\psi$ mesons just above threshold. Early extractions disagree on whether mass is more concentrated than charge.
Precise statement
Determine the gluon gravitational form factors $A_{g}(t)$, $B_{g}(t)$, $D_{g}(t)$ for $-t < 2\,\mathrm{GeV}^{2}$ and the resulting mass and scalar radii. Near-threshold $J/\psi$ photoproduction data from GlueX and JLab Hall C (2019 to 2023) were interpreted with holographic and GPD models giving different radii. Answer: a model-independent extraction, or a demonstration that threshold $J/\psi$ production cannot determine these form factors at 10 percent accuracy.
What would settle it
Lattice QCD calculation of $A_{g}$ and $D_{g}$ at physical pion mass compared with threshold $J/\psi$ and $\Upsilon$ production at JLab and the EIC, with the reaction mechanism (two-gluon exchange versus open-charm intermediate states) fixed by data.
Status in the literature
Unverified note
Extractions from the 2023 JLab data depended on whether the reaction proceeds through two-gluon exchange or open-charm loops, which remained unresolved as of 2026.