In a groundbreaking study led by Joseph Karpie at the Thomas Jefferson National Accelerator Facility, nuclear physicists have made significant progress in unraveling the mystery of how protons acquire their spin. By combining experimental data with cutting-edge calculations, researchers have gained a clearer understanding of the role played by gluons, the "glue" that holds protons together. Moreover, this breakthrough opens the door to visualizing the proton's three-dimensional structure.

The investigation traces its origins back to 1987 when physicists began measuring the sources of a proton's spin. Initially, it was believed that the proton's quarks, its basic building blocks, would be the primary contributors. However, subsequent findings revealed that quarks only account for about 30% of the measured spin. The remaining spin arises from two more elusive sources.

One of these sources is the powerful and enigmatic strong force, one of the fundamental forces in the universe. It is the strong force that binds quarks together, forming subatomic particles like protons and neutrons. Gluons, which are manifestations of this force, were suspected to contribute to the proton's spin. The study also suggests that the movements of quarks and gluons within the proton contribute to its overall spin.

The inspiration for this research was a perplexing result obtained from experimental measurements of gluon spin at the Relativistic Heavy Ion Collider in New York. The initial data seemed to indicate a positive spin contribution from gluons. However, as the analysis improved, another possibility emerged. The updated results suggested that the gluons' spins could have an overall negative contribution, implying that the spin of quarks and their movements might play a more significant role.

A team of physicists from the Jefferson Lab Angular Momentum (JAM) collaboration, alongside the HadStruc collaboration, addressed these measurements in different ways. The HadStruc team employed supercomputers to carry out sophisticated calculations based on Quantum Chromodynamics (QCD), the underlying theory describing quarks' and gluons' interactions in protons. These computational calculations, known as lattice QCD, provided further insights into the puzzle.

Karpie spearheaded the effort to integrate the data from experimental measurements with the lattice QCD results. By combining both sets of data, a more comprehensive and informed understanding emerged. This novel approach demonstrated the value of combining experimental data and theoretical calculations, resulting in a more precise analysis of the gluons' contribution to the proton's spin.

The findings of this study, recently published in Physical Review D, highlight the significance of this collaborative effort. Karpie emphasized that this is just the initial step, and further improvements to the datasets are necessary. As more advanced experiments yield more detailed information about the proton, a more comprehensive understanding of its three-dimensional structure will be developed.

David Richards, a senior staff scientist at the Jefferson Lab, expressed the belief that this research will contribute to the construction of a three-dimensional image of a proton by gradually working towards solving simpler one-dimensional scenarios. The ultimate goal is to gain deeper insights into the heart of the problem by continuously refining methods and techniques.

This breakthrough study on gluon contributions to proton spin marks a significant milestone in nuclear physics, opening up new avenues for further exploration and understanding of one of the fundamental building blocks of matter.