Proton And Neutron Structure

"Nucleons" are protons and neutrons. 

A new paper determines that their mass is distributed over a radius of consistent with one femtometer (a 10^-15 meter), and is spread over a larger area than their electromagnetic charges which are found in their valence quarks. The proton charge radius is 0.842 femtometers. This is because the electromagnetically neutral gluons which bind these quarks, whose energy is the source of most of the mass of protons and neutrons, are spread out more than the electromagnetically charged quarks in a proton or neutron. 

Being closely connected to the origin of the nucleon mass, the gravitational form factors of the nucleon have attracted significant attention in recent years. We present the first model-independent precise determinations of the gravitational form factors of the pion and nucleon at the physical pion mass, using a data-driven dispersive approach. 

The so-called ``last unknown global property'' of the nucleon, the D-term, is determined to be −(3.38+0.26−0.32). The root mean square radius of the mass distribution inside the nucleon is determined to be 0.97+0.02−0.03 fm. Notably, this value is larger than the proton charge radius, suggesting a modern structural view of the nucleon where gluons, responsible for most of the nucleon mass, are distributed over a larger spatial region than quarks, which dominate the charge distribution. We also predict the nucleon angular momentum and mechanical radii, providing further insights into the intricate internal structure of the nucleon.

Xiong-Hui Cao, Feng-Kun Guo, Qu-Zhi Li, De-Liang Yao, "Precise Determination of Nucleon Gravitational Form Factors" arXiv:2411.13398 (November 20, 2024).