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Proton Spin is Clarified Through the Combination of Theory and Experiment

For a long time, nuclear physicists have been trying to figure out how the proton acquires its spin. Recent advances in computational techniques coupled with experimental data have provided a more comprehensive understanding of the spin contributions from the very material that holds protons together. It also opens the door to 3D structure imaging of the proton.

Joseph Karpie, a postdoctoral associate at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility’s Center for Theoretical and Computational Physics (Theory Center), oversaw the project.

He said that observations of the proton’s spin sources in 1987 marked the beginning of this decades-long puzzle. At first, physicists believed that the quarks that make up a proton would be the primary cause of its spin. However, that was not their discovery. It turned out that only roughly 30% of the observed spin of the proton is produced by its quarks. The remaining two sources, which are harder to quantify thus far, provide the remaining amount.

The first is the enigmatic yet potent strong force. One of the universe’s four fundamental forces is the strong force. It is the process that “glues” quarks together to form protons and neutrons, two additional subatomic particles. Gluons are strong force manifestations that are believed to contribute to the spin of the proton. The quarks and gluons in the proton are assumed to be responsible for the remaining spin.

“This paper is sort of a bringing together of two groups in the Theory Center who have been working toward trying to understand the same bit of physics, which is how do the gluons that are inside of it contribute to how much the proton is spinning around,” he explained.

He claimed that a perplexing finding from preliminary experimental measurements of the spin of the gluons served as the impetus for this investigation. The measurements were conducted at the DOE Office of Science user facility, the Relativistic Heavy Ion Collider, located at Brookhaven National Laboratory in New York. Initially, the findings seemed to suggest that the spin of the proton might be influenced by the gluons. They displayed a favorable outcome.

However, as the data analysis was refined, an additional avenue became apparent.

“When they improved their analysis, they started to get two sets of results that seemed quite different, one was positive and the other was negative,” Karpie stated.

The enhanced study allowed for the possibility that the gluons’ spins have an overall negative impact, even if the earlier affirmative result showed that the gluons’ spins are aligned with the proton’s. In that scenario, the motion of the quarks and gluons, or the spin of the quarks themselves, would account for a greater portion of the proton spin.

The Jefferson Lab Angular Momentum (JAM) cooperation issued this perplexing result.

The HadStruc cooperation, meanwhile, had been approaching the same measurements from a different angle. They were calculating Quantum Chromodynamics (QCD), the fundamental theory that explains the interactions between quarks and gluons in the proton, using supercomputers.

Some parts of the theory have been somewhat simplified by theorists in order to enable supercomputers to perform this complex calculation. We refer to this computer-friendly, somewhat simplified variant as lattice QCD.

Karpie oversaw the process of combining the data from the two groups. He began with the aggregated data from studies conducted in global institutions. The outcomes of the lattice QCD computation were then incorporated into his study.

Working on the study was senior staff scientist David Richards of Jefferson Lab. “This is putting everything that we know about quark and gluon spin and how gluons contribute to the spin of the proton in one dimension,” Richards said.

“When we did, we saw that the negative things didn’t go away, but they changed dramatically. That meant that there’s something funny going on with those,” said Karpie.

The work, which was just published in Physical Review D, was led by Karpie. The key lesson, according to him, is that integrating the information from both methods produced a more knowledgeable outcome.

“We’re combining both of our datasets together and getting a better result out than either of us could get independently. It’s really showing that we learn a lot more by combining lattice QCD and experiment together in one problem analysis,” according to Karpie. “This is the first step, and we hope to keep doing this with more and more observables as well as we make more lattice data.”

Enhancing the datasets even more is the next stage. Data start to construct a picture that transcends one dimension as more potent experiments yield more precise information about the proton. Additionally, theorists’ solutions becoming more accurate and comprehensive as they discover how to perform better computations on ever-more potent supercomputers.

Ultimately, the aim is to generate a three-dimensional model of the proton’s structure.

“So, we learn our tools do work on the simpler one-dimension scenario. By testing our methods now, we hopefully will know what we need to do when we want to move up to do 3D structure,” Richards stated. “This work will contribute to this 3D image of what a proton should look like. So it’s all about building our way up to the heart of the problem by doing this easier stuff now.”

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