Nuclear theorists use supercomputers to map the building blocks of matter in 3D

Nuclear theorists from the HadStruc Collaboration have been working on a mathematical description of parton interactions using supercomputers, including machines in Jefferson Laboratory’s Data Center. Credit: Jefferson Laboratory/Eileen Devlin
Deep within what we perceive as solid matter, the landscape is far from static: The interiors of the particles called hadrons that make up atomic nuclei are made up of a fiery mix of interacting quarks and gluons (collectively known as partons), which high school students might recognize as protons and neutrons.
A group of physicists has now come together to understand these partons and how they interact to form hadrons. Based at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility and known as the HadStruc Collaboration, these atomic physicists are working on a mathematical description of parton interactions. Their latest findings were recently published in the Journal of High Energy Physics.
“The HadStruc Collaboration is a group based at Jefferson Institute’s Theory Center and several nearby universities,” said Joseph Karpee, a postdoctoral researcher at Jefferson Institute’s Center for Theoretical and Computational Physics and a HadStruc member. “We also have members at the College of William and Mary and Old Dominion University.”
Other co-authors on the paper are Jefferson Institute scientists Robert Edwards, Collin Egeler, Eloy Romero and David Richards. The College of William & Mary Physics Department is headed by Hervé Dutriou, Christopher Monahan and Kostas Orginos, who also holds a joint appointment at Jefferson Institute. Anatoly Radushkin is also an associate faculty member at Jefferson Institute, affiliated with Old Dominion University, and Savvas Zafeiropoulos is at the University of Toulon in France.
Powerful theory
The partons (particles) that make up hadrons are held together by the strong interaction, which is one of the four fundamental forces of nature, along with gravity, electromagnetic force, and the weak force observed in particle decays.
Carpier explained that, like many theoretical physicists around the world, members of the HadStruc collaboration are trying to figure out where and how quarks and gluons are distributed inside a proton. The group uses a mathematical technique called lattice quantum chromodynamics (QCD) to calculate the structure of the proton.
Dutriou, a postdoctoral researcher at the College of William and Mary, explained that the group’s paper outlines a three-dimensional approach to understanding hadron structure through a QCD lens, which was then implemented through supercomputer calculations.
The 3D concept is based on the notion of generalized parton distributions (GPDs), which have theoretical advantages over structures visualized through the traditional QCD approach: 1D parton distribution functions (PDFs).
“Well, GPD is much better in the sense that it can answer one of the big questions about the proton: how does spin come about,” Dutrieux says. “One-dimensional PDFs give us very limited information on that.”
He explained that the proton is made up of, to the first approximation, two up quarks and one down quark (called a valence quark). The valence quarks act to glue the quarks together, mediated by various gluons generated from the strong force interaction. These gluons, as well as quark-antiquark pairs (usually represented as a sea of ​​quarks-antiquarks to distinguish them from valence quarks), are continually generated and reintegrated into the strong force.
One of the surprising discoveries about the spin of the proton was made in 1987, when experimental measurements demonstrated that the spin of quarks contributes less than half of the total spin of the proton. In fact, much of the spin of the proton may arise from the spin of gluons and the motion of the partons in the form of orbital angular momentum. Much experimental and computational effort is still needed to clarify this situation.
“GPD represents a promising opportunity to access this orbital angular fraction and generate a well-founded explanation of how the proton’s spin is distributed among quarks and gluons,” Dutriou noted.
He went on to say that another aspect the collaboration hopes to address through GPD is a concept known as the energy-momentum tensor.
“The energy-momentum tensor actually tells us how the energy and momentum are distributed inside the proton,” Dutriou says, “and also how the proton interacts with gravity. But at the moment we’re just studying the distribution of matter.”


A brief summary of the extraction of GPD Mellin moments using the dipole representation. The uncertainties cited include an assessment of statistical and excited state uncertainties. Credit: Journal of High Energy Physics (2024). DOI: 10.1007/JHEP08(2024)162
Simulating Data
As mentioned above, accessing this information requires advanced calculations using supercomputers. After developing their new approach, the theorists carried out 65,000 simulations to test the theory and its assumptions.
This massive number of calculations were performed on the Frontera supercomputer at the Texas Advanced Computing Center and the Frontier supercomputer at the Oak Ridge Leadership Computing Facility, a DOE Office of Science User Facility at Oak Ridge National Laboratory. The number included 186 simulations of protons moving with different momenta against the backdrop of a randomly generated set of 350 gluons. In total, the calculations required processors at these facilities to run for millions of hours. Final analysis of these results will be completed on a smaller supercomputer at Jefferson Laboratory.
The result of this study was a robust test of the 3D approach developed by the theorists, a test that represents an important milestone for DOE’s Quark-Gluon Tomography (QGT) Topical Collaboration.
“This was a proof of principle. We wanted to know if the results of these simulations were reasonable based on what we already know about these particles,” Karpi said. “The next step is to improve the approximations we used in these calculations, which are 100 times more computationally expensive in terms of computation time.”
New data is on the way
Carpi noted that the HadStruc Collaboration’s GPD theory is already being tested in experiments at high-energy facilities around the world, including at Jefferson Laboratory, where two processes that probe the structure of hadrons through GPD are being performed: deep virtual Compton scattering (DVCS) and deep virtual meson production (DVMP).
Carpier and Dutriou hope that the group’s work will be included in planned experiments at the Electron Ion Collider (EIC), a particle accelerator under construction at the Department of Energy’s Brookhaven National Laboratory on Long Island, with which Jefferson Laboratory is partnering on the project.
It is hoped that the EIC will be powerful enough to study hadrons beyond the point where current instruments begin to lose signal, but investigations into the structure of how hadrons are put together need not wait until the EIC is online.
“There are some new experiments going on at Jefferson Laboratory. They’re collecting data right now, giving us information to compare with our calculations,” Karpi said. “And we’re hoping to accumulate and have even better information available at the EIC. It’s all part of a chain of progress.”
Members of the HadStruc Collaboration aim to further experimental applications of QCD theory research at Jefferson Laboratory and other facilities, including using supercomputers to calculate more precise results from data that has been accumulating for decades.
Karpi added that he hopes to stay a few steps ahead of the experimenters.
“QCD has always lagged behind experiment, and we’ve usually been ‘predicting’ what’s going on rather than ‘predicting’ it,” Karpy says, “so if we can actually get ahead of it – if we can do something that experimentalists can’t do yet – that would be pretty awesome.”
Further information: Hervé Dutrieux et al. “Towards non-polarizable GPDs from pseudodistributions.” Journal of High Energy Physics (2024). DOI: 10.1007/JHEP08(2024)162
Courtesy of Thomas Jefferson National Accelerator Facility
Citation: Nuclear theorists use supercomputers to map the building blocks of matter in 3D (September 17, 2024) Retrieved September 18, 2024 from https://phys.org/news/2024-09-nuclear-theorists-supercomputers-blocks-3d.html
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