Determining the Gluonic Gravitational Form Factors of the Proton

Journal Article · Thu Mar 30 00:00:00 EDT 2023 · Nature (London)

DOI:https://doi.org/10.1038/s41586-023-05730-4· OSTI ID:2000517

Meziani, Z. -E.; Duran, B.; Joosten, S.; Jones, M. K.; Prasad, S.; Peng, C.; Armstrong, W.; Atac, H.; Chudakov, E.; Bhatt, H.; Reimer, P. E.; Xie, J.; Ye, Zhenyu

The proton is one of the main building blocks of all visible matter in the Universe1. Among its intrinsic properties are its electric charge, mass and spin2. These properties emerge from the complex dynamics of its fundamental constituents—quarks and gluons—described by the theory of quantum chromodynamics3,4,5. The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering2. An example is the highly precise measurement of the electric charge radius of the proton6. By contrast, little is known about the inner mass density of the proton, which is dominated by the energy carried by gluons. Gluons are hard to access using electron scattering because they do not carry an electromagnetic charge. Here we investigated the gravitational density of gluons using a small colour dipole, through the threshold photoproduction of the J/ψ particle. We determined the gluonic gravitational form factors of the proton7,8 from our measurement. We used a variety of models9,10,11 and determined, in all cases, a mass radius that is notably smaller than the electric charge radius. In some, but not all cases, depending on the model, the determined radius agrees well with first-principle predictions from lattice quantum chromodynamics12. This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter.The proton is one of the main building blocks of all visible matter in the Universe(1). Among its intrinsic properties are its electric charge, mass and spin(2). These properties emerge from the complex dynamics of its fundamental constituents-quarks and gluons-described by the theory of quantum chromodynamics(3-5). The electric charge and spin of protons, which are shared among the quarks, have been investigated previously using electron scattering(2). An example is the highly precise measurement of the electric charge radius of the proton(6). By contrast, little is known about the inner mass density of the proton, which is dominated by the energy carried by gluons. Gluons are hard to access using electron scattering because they do not carry an electromagnetic charge. Here we investigated the gravitational density of gluons using a small colour dipole, through the threshold photoproduction of the J/? particle. We determined the gluonic gravitational form factors of the proton(7,8) from our measurement. We used a variety of models(9-11) and determined, in all cases, a mass radius that is notably smaller than the electric charge radius. In some, but not all cases, depending on the model, the determined radius agrees well with first-principle predictions from lattice quantum chromodynamics(12). This work paves the way for a deeper understanding of the salient role of gluons in providing gravitational mass to visible matter.

• Research Organization: Argonne National Laboratory (ANL), Argonne, IL (United States)
• Sponsoring Organization: USDOE Office of Science - Office of Nuclear Physics
• DOE Contract Number: AC02-06CH11357
• OSTI ID: 2000517
• Journal Information: Nature (London), Vol. 615, Issue 7954
• Country of Publication: United States
• Language: English