New Approach Merges Theoretical Fundamentals with Experimental Studies of the Proton’s Structure

A new approach to applying quantum chromodynamics paves the way for a deeper understanding of the strong nuclear interaction.

Artist’s depiction of a proton’s interior. In quantum chromodynamics, the constituent quarks come in three “colors,” along with up and down “flavors.” Also shown are virtual quark-antiquark pairs and the gluons that bind the quarks together.
Image courtesy of Ted Rogers
Artist’s depiction of a proton’s interior. In quantum chromodynamics, the constituent quarks come in three “colors,” along with up and down “flavors.” Also shown are virtual quark-antiquark pairs and the gluons that bind the quarks together.

The Science

Protons and other subatomic particles that are subject to the strong nuclear force have a complex structure that involves even more fundamental constituents called quarks and gluons. These quarks and gluons bind under the influence of quantum chromodynamics (QCD). QCD is the theory of strong interaction of quarks and the role of color symmetry. However, the mechanisms that lead to quarks and gluons combining to form the particles we see in nature are very mysterious and poorly understood. For example, virtual quarks and gluons constantly appear and disappear within our current picture of the dynamics in the proton. So, which quarks and gluons are actually “in” a proton is a difficult question to answer.  A major part of QCD research is driven by the goal of solving open fundamental questions at the heart of how QCD operates and how quantum mechanical theories can be merged with the theory of relativity. Recent progress in theoretical QCD has opened the way to connecting questions about the structure of particles like the proton with high-energy particle collision measurements.

The Impact

As theorists generate new ideas about QCD, other researchers plan experiments to test those ideas. These tests involve colliding particles like electrons and protons at high energies and then examining the results. By extrapolating backwards in time, physicists will use the remnants of the collisions to infer information about the structure of the original particles. However, the same theoretical difficulties that motivated these studies have left a key question unresolved. Namely, how do scientists relate the physics of the specific collisions with the physics of the internal structure of the particles themselves? The recent work provides the toolbox needed to resolve this question while also accounting for the theoretical subtleties.

Summary

Much of the experimental work related to extracting the quark and gluon structure of protons occurs at existing particle accelerators like the Thomas Jefferson National Accelerator Facility and the Relativistic Heavy Ion Collider, and in the future at the Electron Ion Collider. A large part of current research into the structure of the proton, both theoretical and experimental, involves identifying, extracting, and analyzing the bound state distributions of quarks and gluons in the proton, mapping out their motion, and understanding how this relates to the overall observed properties of the proton like its spin and mass. In the past, researchers found inconsistencies in the way physicists combined fundamental QCD theory to the study of data. The new theoretical results provide a clear recipe and boost confidence that data taken in future experiments can be reliably interpreted.

Contact

Ted Rogers
Old Dominion University and Thomas Jefferson Accelerator Facility 
[email protected]

Funding

This work was supported by the Department of Energy (DOE) Office of Science, Office of Nuclear Physics. This work was also supported by the DOE contract under which Jefferson Science Associates, LLC operates the Thomas Jefferson National Accelerator Facility. Support was also provided by the European Union’s Horizon 2020 research and innovation program.

Publications

Aslan, F., Boglione, M., Gonzalez-Hernandez, J.O., Rainaldi, T., Rogers, T. C., and Simonelli, A., Phenomenology of TMD parton distributions in Drell-Yan and Z0 boson production in a hadron structure oriented approach. Physical Review D 110, 7 (2024). [DOI: 10.1103/PhysRevD.110.074016]

Gonzalez-Hernandez, J. O., Rainaldi, T., and Rogers, T. C., The Resolution to the Problem of Consistent Large Transverse Momentum in TMDs. Physical Review D 107, 9 (2023). [DOI: 10.1103/PhysRevD.107.094029]

Gonzalez-Hernandez, J. O., Rogers, T. C., and Sato, N., Combining Nonperturbative Transverse Momentum Dependence with TMD Evolution. Physical Review D 106, 3 (2022). [DOI: 10.1103/PhysRevD.106.034002]

Related Links

Learn more at the Hadron Structure Oriented Approach to Transverse Momentum Dependents site.

Highlight Categories

Program: NP

Performer: University , DOE Laboratory , RHIC