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Scientists locate missing mass inside proton



Newswise — The way mass is distributed in a proton is a fundamental enigma in nuclear physics and particle physics. A proton’s nucleus is made up of three valence quarks, but they contribute only a small fraction of the proton’s mass. Most mass emerges from the complex dynamics of quarks, including their movement, and is primarily governed by the a powerful force mediated by gluons. Scientists have made a new measurement that represents a major advance in their understanding of the mass of the proton. This measurement located the central proton region as the main source of mass generated by gluons.

The impact

For years, nuclear physicists have estimated the size of the proton through precise measurements of its electrical charge response. This is the result of the proton’s electrically charged constituent quarks. However, determining the size of matter in terms of proton size is a more difficult challenge. Indeed, part of the proton’s mass is not determined by the mass or motion of its charged quarks, but rather by the elusive neutral gluons. These gluons bind the quarks and themselves within the proton. A new discovery offers a view of this region of mass generated by gluon interactions. This measurement not only reveals the resulting mass radius of the strong force, but also unveils its confining influence on quarks, which extends well beyond the proton’s electric charge radius.


Nuclear physicists have studied the details of the proton’s structure since its discovery in 1917. An important detail of the proton’s structure is its size. The most commonly used metric to measure the size of the proton is its charge radius, which uses electrons to measure the spherical size of the proton’s electrical charge. This new measurement comes from the J/Ψ -007 experiment at the Thomas Jefferson National Accelerator Facility. It is different in that it used a small color dipole (the J/Psi particle) to reveal a spherical size and location of the gluon mass and its sphere of influence on the gluons in the proton.

The scientists made these measurements using the Continuous Electron Beam Accelerator Facility, a Department of Energy Office of Science User Facility. In the experiment, scientists used a beam of energetic electrons to produce J/Ψ particles from protons. The J/Ψ particles provide information on the distribution of gluons inside the proton. The experimenters inserted these measurements into theoretical models for analysis. The result was a determination of the mass radius of the gluon inside the proton. Furthermore, this also indicated a strong force sphere of influence, called the confining scalar cloud, which also affects the proton’s quarks.


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

Journal link: Nature, March 2023


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