Nucleon Strong-Interactions Size From Charmonium Photoproduction

Abstract

Protons and neutrons differ dramatically in their electromagnetic sizes, but are bound by the strong (color) force and have nearly identical masses, like two twin states of the same ``nucleon". To reveal the overall spatial extent, it must be examined through the strong interaction rather than electromagnetism. The $J/ψ$ particle, a compact color-charge dipole of a charm quark and antiquark, provides an ideal probe. Recent high-precision measurements of near-threshold $J/ψ$ production at Jefferson Lab offer a more precise glimpse of the proton's gluonic spatial structure. Combining these data with recent quark-sector results, we extract the distribution of the color field underlying mass generation in the nucleon and find it extends beyond both charge and mass distributions, revealing the nucleon's overall strong-interaction spatial extent of $0.94\pm 0.09$ fm.