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.
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…
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