Speaker
Description
Understanding the origin of the proton's mass is a fundamental question in nuclear physics. The proton’s three valence quarks account for only a small fraction of its total mass, with the majority arising from the strong force, mediated by gluons.
Recent experiments studying near-threshold J/ψ production at Jefferson Lab have provided new insights into the proton's mass distribution. I will highlight key results from the J/ψ-007 experiment in Hall C, which performed the first measurement of the differential cross-section for J/ψ photoproduction near threshold. This analysis enabled the determination of the proton’s gluonic gravitational form factors (GFFs) directly from experimental data. The J/ψ-007 results revealed that the proton's mass radius is smaller than its charge radius, with evidence for a scalar gluon cloud extending beyond the charge radius to about 1 femtometer.
The focus of this talk will be on new results from the J/ψ → μ⁺μ⁻ decay channel, which effectively doubles the 2023 dataset and significantly improves constraints on the gluonic GFFs. This expanded dataset allows for the extraction of pressure and shear distributions of gluons within the proton, offering deeper insights into its internal mass and mechanical structure. I will also preview the forthcoming experimental program with SoLID at Jefferson Lab and ePIC at the Electron-Ion Collider.
