The science

Newswise – The way mass is distributed in a proton is a fundamental puzzle in nuclear and particle physics. The nucleus of a proton consists of three valence quarks, but these only make up a small fraction of the proton’s mass. Most of the mass arises from the complicated quark dynamics, including their motion, and is mainly determined by the quark dynamics strong force mediated by gluons. Scientists have made a new measurement that represents a breakthrough in their understanding of the proton’s mass. This measurement pinpointed the central region of the proton as the primary source of mass produced by gluons.

The impact

For years, nuclear physicists have measured the size of the proton through precise measurements of its electrical charge response. This is due to the electrically charged quarks that make up the proton. However, determining the size of matter based on proton size is a more difficult challenge. This is because part of the proton’s mass is determined not by the mass or motion of its charged quarks, but by the elusive neutral gluons. These gluons bind the quarks and themselves in the proton. A new finding provides insight into this mass range created by the interactions of gluons. This measurement not only deciphers the mass radius resulting from the strong force, but also reveals its constraining influence on the quarks, which goes far beyond the proton’s electric charge radius.


Nuclear physicists have uncovered details about the structure of the proton since its discovery in 1917. An important detail of the proton’s structure is its size. The most commonly used measure of the proton’s size is its charge radius, which uses electrons to measure the spherical size of the proton’s electric charge. This new measurement comes from the J/Ψ-007 experiment at the Thomas Jefferson National Accelerator Facility. The difference is that a small color dipole (the J/Psi particle) was used to reveal the spherical size and location of the mass of the gluons, as well as their area of ​​influence on the gluons in the proton.

Scientists conducted these measurements using the Continuous Electron Beam Accelerator Facility, a user facility of the Department of Energy Office of Science. In the experiment, scientists used a beam of energetic electrons to create J/Ψ particles from protons. The J/Ψ particles provide information about the distribution of the gluons in the proton. The experimenters inserted these measurements into theoretical models for analysis. The result was a determination of the mass radius of the gluon in the proton. In addition, it also suggested an area of ​​​​influence of strong forces, the so-called limiting scalar cloud, which also influences the quarks of the proton.


This work was supported in part by the Department of Energy Office of Science and the Office of Nuclear Physics.

Journal link: Nature, March 2023

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