Theory and experiment combine to shed new light on proton spin

Nuclear physicists have long strived to discover how the proton gets its spin. Now, a new method combining experimental data with cutting-edge calculations has revealed a more detailed picture of the spin contributions from the very glue that holds protons together. This also paves the way for imaging the 3D structure of the proton.

The work was led by Joseph Karpie, a postdoctoral associate in the Center for Theoretical and Computational Physics (Theory Center) at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility.

He explained that this decades-old mystery began with measurements of the sources of the proton’s spin in 1987. Physicists originally thought that the proton’s building blocks, its quarks, would be the main source of the proton’s spin. But that’s not what they found. It turned out that the proton’s quarks provide only about 30% of the proton’s total measured spin. The rest comes from two other sources that have so far proven more difficult to measure.

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

“This paper is sort of the bringing together of two groups at the Theory Center who have been working to try to understand the same piece of physics, which is how do the gluons that are there contribute to the rotation of the proton,” he said. said.

He said this study was inspired by a puzzling result from early 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 proton’s spin. They showed a positive result.

But as data analysis improved, another possibility emerged.

“As they improved their analysis, they started getting two sets of results that looked very different, one positive and one negative,” Karpie explained.

While the previous positive result indicated that the gluon spins are aligned with that of the proton, the improved analysis allowed the possibility that the gluon spins have an overall negative contribution. In this case, more of the proton’s spin would come from the movement of 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 had approached the same measures in a different way. They used supercomputers to calculate the underlying theory describing the interactions between quarks and gluons in the proton, quantum chromodynamics (QCD).

To equip supercomputers to perform this intense calculation, theorists somewhat simplify certain aspects of the theory. This somewhat simplified version for computers is called network-based QCD.

Karpie led the work to bring together data from the two groups. It began with combined data from experiments performed at facilities around the world. He then added the results of the lattice QCD calculation to his analysis.

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

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

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

“We combine our two data sets and get a better result than either of us could get independently. It really shows that we learn a lot more by combining network QCD and experimenting together in a single problem analysis,” Karpie said . “This is the first step, and we hope to continue down this path with more and more observables and generating more network 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 a single dimension. And as theorists learn to improve their calculations on ever more powerful supercomputers, their solutions also become more precise and more inclusive.

The objective is to ultimately achieve a three-dimensional understanding of the structure of the proton.

“So we’re learning 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 the 3D structure,” Richards said. “This work will contribute to this 3D picture of what a proton should look like. So it’s about getting to the heart of the problem by making these things easier now.”