Joseph Karpie, a postdoctoral associate at the Center for Theoretical and Computational Physics at the U.S. Department of Energy's Thomas Jefferson National Accelerator Facility, led the research.
He explained that understanding proton spin has been a puzzle since 1987. Initially, physicists thought quarks were the primary source of proton spin, but quarks only account for about 30%. The remainder comes from the strong force and the movements of quarks and gluons.
"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," Karpie said.
The study was inspired by results from initial measurements of gluon spin at the Relativistic Heavy Ion Collider, a DOE Office of Science user facility at Brookhaven National Laboratory. These measurements showed positive results, suggesting that gluons contribute to proton spin. However, improved analysis revealed two sets of differing results, one positive and one negative.
"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 explained.
This conflicting result was published by the Jefferson Lab Angular Momentum (JAM) collaboration. Meanwhile, the HadStruc collaboration addressed the same measurements using supercomputers to calculate Quantum Chromodynamics (QCD), the theory describing interactions among quarks and gluons.
Karpie integrated data from global experiments with lattice QCD calculations.
"This is putting everything together that we know about quark and gluon spin and how gluons contribute to the spin of the proton in one dimension," said David Richards, a Jefferson Lab senior staff scientist.
Combining the datasets provided a more informed result. "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," Karpie said. "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."
The next step is to improve datasets further. As more powerful experiments yield detailed information, these data will help create a 3D understanding of proton structure.
"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," Richards said.
Research Report:Gluon helicity from global analysis of experimental data and lattice QCD Ioffe time distributions
Related Links
Thomas Jefferson National Accelerator Facility
Understanding Time and Space
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