Protons are one of the main building blocks of all visible matter in the universe. Its unique properties include charge, mass, and spin. These properties emerge from the complex dynamics of its basic building blocks, quarks and gluons, explained by the theory of quantum chromodynamics. The charge and spin of protons shared between quarks has been previously studied using electron scattering. One example is the high-precision measurement of the charge radius of protons. In contrast, little is known about the internal mass density of protons, which is dominated by the energy carried by gluons. In a new study, a team of physicists led by Argonne National Laboratory used a small colored dipole to probe the gravitational density of gluons through threshold photogeneration of J/ψ (J/Psi) particles.
Proton valence quarks (blue, red, green), quark and antiquark pairs, and gluons (springs). Scalar gluon activity (pink) extends beyond the charge radius (orange) surrounding the gluon energy core (yellow). Image credit: Argonne National Laboratory.
For many years, nuclear physicists have determined the size of protons by precisely measuring their charge response. This is a result of the proton's charged constituent quarks.
However, determining the size of matter by the size of its protons is a more difficult task. This is because part of the proton's mass is driven by the elusive neutral gluon, rather than by the mass or motion of charged quarks. These gluons combine themselves with quarks within the proton.
The new discovery provides a view of this mass region produced by gluon interactions.
This measurement not only reveals the mass radius resulting from the strong force, but also its confinement effect on quarks that extend far beyond the proton's charge radius.
“A key detail of the proton's structure is its size,” said lead author Dr. Zein Eddin Meziani, a physicist at Argonne National Laboratory, and his colleagues.
“The most commonly used measure of a proton's size is its charge radius, which uses electrons to measure the spherical size of the proton's charge.”
The new measurements come from the J/Ψ -007 experiment at the Thomas Jefferson National Accelerator Facility.
This differs in that a small colored dipole ( ) was used to reveal the sphere size and position of the gluon mass and its range of influence on the gluon within the proton.
In the experiment, physicists used a high-energy beam of electrons to create J/Ψ particles from protons. The J/Ψ particle provides information about the distribution of gluons inside the proton.
Experimenters inserted these measurements into a theoretical model and analyzed them.
As a result, the mass radius of the gluon inside the proton was determined.
Furthermore, the area of influence of a strong force called a confinement scalar cloud, which also affects proton quarks, was also shown.
“This study paves the way for a deeper understanding of the prominent role of gluons in imparting gravitational mass to visible matter,” the authors concluded.
Their paper It was published in the magazine Nature.
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B. Duran other. 2023. Determination of the Grunick gravitational shape factor of protons. Nature 615, 813-816; doi: 10.1038/s41586-023-05730-4
Source: www.sci.news
