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Nuclear physicists have long worked to uncover how the proton gets its spin. Now a new method that combines experimental data with state-of-the-art calculations has revealed a more detailed picture of the spin contribution from the very glue that holds protons together. It also paves the way to imaging the 3D structure of the proton.

The work was led by Joseph Carpi, a postdoctoral fellow in the Center for Theoretical and Computational Physics (Theoretical Center) at the US Department of Energy’s Thomas Jefferson National Accelerator.

He said this decades-old mystery began with measurements of the proton’s spin sources in 1987. Physicists originally thought that the proton’s building blocks, its quarks, would be the main source of the proton’s spin. But they didn’t find that. It turns out that the quarks on the proton provide only about 30% of the total measured spin of the proton. The rest come from two other sources that have so far proved more difficult to measure.

One is the mysterious but powerful strong force. The strong force is one of the four fundamental forces in the universe. This is what “sticks” quarks together to form other subatomic particles, such as protons or neutrons. Manifestations of this strong force are called gluons, which are thought to contribute to the spin of the proton. The latter part of the spin is thought to come from the motions of the proton’s quarks and gluons.

“This paper is sort of a coming together of two groups at the Theory Center that are working on trying to understand the same part of physics, which is how the gluons that are inside it contribute to how much the proton spins around,” he said. .

He said this research was inspired by a puzzling result that came from initial experimental measurements of gluon spin. The measurements were made at the Relativistic Heavy Ion Collider, a DOE Office of Science user facility based at Brookhaven National Laboratory in New York. At first, the data seemed to indicate that gluons might contribute to the spin of the proton. They showed a positive result.

But as data analysis has improved, an additional opportunity has emerged.

“When they improved their analysis, they started getting two sets of results that looked quite different, one was positive and the other was negative,” Carpi explained.

While the earlier positive result indicated that the spins of the gluons are aligned with those of the proton, the improved analysis allows for the possibility that the spins of the gluons have an overall negative contribution. In this case, more of the proton spin will come from the motion of the quarks and gluons, or from the spin of the quarks themselves.

This puzzling result was published by the Jefferson Lab Angular Momentum (JAM) collaboration.

Meanwhile, the HadStruc collaboration dealt with the same measurements in a different way. They used supercomputers to calculate the fundamental theory that describes the interactions between quarks and gluons in the proton, quantum chromodynamics (QCD).

To equip supercomputers to do this intensive calculation, theorists somewhat simplified some aspects of the theory. This somewhat simplified version for computers is called lattice QCD.

Karpie led the work to merge the data from the two groups. He started with the combined data from experiments done at facilities around the world. He then added the results of the lattice QCD calculation into his analysis.

“This brings together everything we know about quark and gluon spin and how gluons contribute to proton spin in one dimension,” said David Richards, a senior scientist at Jefferson Lab who worked on the study.

“When we did that, we saw that the negative things didn’t go away, but they changed dramatically. It meant there was something funny going on with them,” Carpi said.

Karpie is the lead author of the study, which was recently published in Physical examination D. He said the main takeaway is that combining the data from both approaches gives a more informed result.

“We combine both sets of data together and get a better result than either of us could get independently. It really shows that we learn a lot more by combining lattice QCD and experiments together in one analysis of the problem,” Carpi said. “This is the first step and we hope to continue to do this with more and more observables as well as doing more gridded data.”

The next step is to further improve the datasets. As more powerful experiments provide more detailed information about the proton, these data begin to paint a picture that goes beyond one dimension. And as theorists learn how to improve their calculations on increasingly powerful supercomputers, their solutions also become more precise and comprehensive.

The goal is to ultimately create a three-dimensional understanding of the proton’s structure.

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