The proton is one of the main building blocks of all visible matter in the Universe. Among its intrinsic properties are its electric charge, mass and spin. These properties emerge from the complex dynamics of its fundamental constituents — quarks and gluons — described by the theory of quantum chromodynamics. The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering. An example is the highly precise measurement of the electric charge radius of the proton. By contrast, little is known about the inner mass density of the proton, which is dominated by the energy carried by gluons. In new research, a team of physicists led by Argonne National Laboratory investigated the gravitational density of gluons using a small color dipole, through the threshold photoproduction of the J/ψ (J/Psi) particle.
A proton’s valence quarks (blue, red, and green), quark-antiquark pairs, and gluons (springs); scalar gluon activity (pink) extends beyond the electric charge radius (orange) that surrounds the gluonic energy core (yellow). Image credit: Argonne National Laboratory.
For years, nuclear physicists have gauged the proton’s size through precise measurements of its electric charge response. This is a result of the proton’s electrically charged constituent quarks.
However, determining the size of the matter in the proton size is a more difficult challenge. This is because a portion of the proton’s mass is not driven 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.
The new finding offers a view of this region of mass generated by the interactions of gluons.
The measurement not only unravels the mass radius resulting from the strong force but also unveils its confining influence on the quarks that extends well beyond the electric charge radius of the proton.
“An important detail of the proton’s structure is its size,” said lead author Dr. Zein-Eddine Meziani, a physicist at Argonne National Laboratory, and colleagues.
“The most commonly used measure for the size of the proton is its charge radius, which uses electrons to measure the spherical size of the proton’s electric charge.”
The 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 ) to reveal a spherical size and location of the gluons’ mass and its sphere of influence on the gluons in the proton.
In the experiment, the physcists 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 gluon’s mass radius inside the proton.
In addition, it also indicated a strong force sphere of influence, called the confining scalar cloud, that also affects the proton’s quarks.
“This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter,” the authors concluded.
Their paper was published in the journal Nature.
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B. Duran et al. 2023. Determining the gluonic gravitational form factors of the proton. Nature 615, 813-816; doi: 10.1038/s41586-023-05730-4
Source : Breaking Science News