Gluons, quarks, and the mystery of what makes a proton spin

For decades, nuclear physicists have been working to uncover the mysterious origins of the proton’s spin. According to a new study, they seem to have finally made some progress.

By combining experimental data with state-of-the-art calculations, researchers have revealed a more detailed picture of the spin contributions from the very glue that holds protons together, paving the way for imaging the proton’s 3D structure.

Puzzling nature of the proton’s spin

The mystery of the proton’s spin began in 1987 when measurements revealed that the proton’s building blocks, its quarks, only provide about 30% of the proton’s total measured spin. This unexpected finding left physicists wondering about the sources of the remaining spin.

Two other potential sources have proven more difficult to measure: the strong force, one of the four fundamental forces in the universe that “glues” quarks together, and the movements of the proton’s quarks and gluons.

Gluons, manifestations of the strong force, are thought to contribute to the proton’s spin, but the exact nature of their contribution has been a topic of ongoing research.

Combining two approaches

This new study was led by postdoctoral associate Joseph Karpie from the Center for Theoretical and Computational Physics at the Thomas Jefferson National Accelerator Facility.

Karpie’s study was inspired by a puzzling result from initial experimental measurements of the gluons’ spin at the Relativistic Heavy Ion Collider, a DOE Office of Science user facility based at Brookhaven National Laboratory.

The data initially seemed to indicate that the gluons may be contributing positively to the proton’s spin, but as the analysis was improved, a negative contribution also became a possibility.

Supercomputers and simplified proton spin theories

Meanwhile, the HadStruc collaboration had been addressing the same measurements using supercomputers to calculate the underlying theory, Quantum Chromodynamics (QCD), that describes the interactions among quarks and gluons in the proton.

To equip supercomputers for this intense calculation, theorists use a somewhat simplified version of QCD called lattice QCD.

Karpie led the work to bring together the data from both groups, combining experimental data from facilities around the world with the results from the lattice QCD calculation.

As David Richards, a Jefferson Lab senior staff scientist who worked on the study, explained, “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.”

Collaboration and dataset combination

The main takeaway from the study, published in Physical Review D, is that combining data from both approaches 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,” said Karpie. “It’s really showing that we learn a lot more by combining lattice QCD and experiment together in one problem analysis.”

The next step is to further improve the datasets as more powerful experiments provide more detailed information on the proton and theorists learn how to improve their calculations on ever-more powerful supercomputers. The goal is to eventually produce a three-dimensional understanding of the proton’s structure.

Richards emphasized the importance of this work in building towards a more comprehensive understanding, saying, “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.”

New era in proton spin research

In summary, the study led by Joseph Karpie has taken a significant step towards unraveling the mystery of the proton’s spin by combining experimental data with state-of-the-art lattice QCD calculations.

This collaborative approach has provided a more detailed picture of the spin contributions from gluons and laid the foundation for future research aimed at imaging the proton’s 3D structure.

As more advanced experiments and calculations become available, researchers will continue to build upon this work, edging closer to a comprehensive understanding of the fundamental building blocks of matter.

The power of collaboration and the integration of diverse datasets have proven to be the key to unlocking the secrets of the proton, and this study serves as a testament to the importance of interdisciplinary efforts in advancing our knowledge of the subatomic world.

The full study was published in the journal Physical Review D.

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