At the fundamental level, what we perceive as solid matter is intricately dynamic. The building blocks of atomic nuclei—hadrons, which include protons and neutrons—are composed of constantly interacting particles known as quarks and gluons. Collectively termed partons, these subatomic entities are at the heart of ongoing research that strives to unveil their interactions and the consequent formation of hadrons. A dedicated group of physicists, known as the HadStruc Collaboration, is at the forefront of this endeavor, probing the intricacies of parton behavior and structure from their base at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility.
The HadStruc Collaboration consists of researchers from various institutions, including Jefferson Lab, William & Mary, and Old Dominion University, among others. Among these physicists, Joseph Karpie leads the way in exploring novel methods to understand how partons contribute to the characteristics of protons and neutrons. The collaboration’s latest findings, recently published in the *Journal of High Energy Physics*, illustrate the power of collaborative research in the complex field of nuclear physics.
Karpie emphasizes the importance of teamwork, indicating that the collaboration’s research is enriched by contributions from varied academic backgrounds and theoretical perspectives. This collaborative approach enables them to tackle challenging questions about the distribution of quarks and gluons in protons, alongside examining how they are glued together by the fundamental strong force.
The primary challenge facing physicists like those in the HadStruc Collaboration is understanding the proton’s internal structure. They utilize a mathematical framework called lattice quantum chromodynamics (QCD) which effectively describes how these subatomic particles interact. This sophisticated approach allows physicists to simulate and analyze interactions at a microscopic level, giving rise to a three-dimensional perspective of hadronic structures.
A significant advancement of their work involves generalized parton distributions (GPDs), which provide a more detailed view of the spatial distribution of partons within protons than previous one-dimensional models. Hervé Dutrieux, a postdoctoral researcher in the collaboration, points out that this enhanced perspective is vital for deciphering perplexities related to the proton’s spin. Notably, experiments have shown that quarks contribute only a fraction of the proton’s spin, indicating a more complex interplay involving gluons and orbital angular momentum.
The proton’s spin has long been a mystery in physics. An iconic moment occurred in 1987 when experiments revealed that the quark spin accounts for less than half of the overall spin of the proton. What appears to contribute significantly to this spin are the spins and movement of gluons, along with the dynamics of partons. The exploration of GPDs provides a promising avenue to comprehend how this distribution of spin occurs among gluons and quarks.
Dutrieux elaborates on another aspect of this research: the energy-momentum tensor. This concept sheds light on how energy and momentum are arranged within the proton, which can extend our understanding of the particle’s interactions with gravity. However, to unlock these insights, intricate and resource-intensive calculations on supercomputers are indispensable.
The research undertaken by the HadStruc Collaboration is backed by extensive simulations conducted on powerful supercomputers. They performed 65,000 simulations to validate their theoretical approach, utilizing the Frontera and Frontier supercomputers—facilities that provide the computational prowess necessary for these demanding calculations. Karpie signifies the value of these simulations: “Our next step is to improve the approximations we used in these calculations,” which will entail a quantum leap in computational demand.
The immense amount of data produced from these simulations requires rigorous analysis, contributing to a deeper understanding of hadronic structures—a milestone that is integral to the Department of Energy’s Quark-Gluon Tomography (QGT) collaboration.
As the HadStruc Collaboration builds on its findings, its work is poised to inform forthcoming experimental efforts at high-energy facilities worldwide. Notably, processes like deeply virtual Compton scattering and deeply virtual meson production are being investigated, with the hope that these experiments will bring the theories of GPDs closer to empirical evidence.
Also on the horizon is the Electron-Ion Collider (EIC) at Brookhaven National Laboratory, which promises increased sensitivity to probing hadronic structures. Unfolding this knowledge isn’t merely about anticipation; it’s part of a continuum of progress that paints a more detailed picture of matter at the subatomic level.
In the evolving realm of particle physics, the HadStruc Collaboration exemplifies a critical balance between theoretical inquiry and experimental validation. By employing advanced computational techniques, they aspire to remain a step ahead, transforming the understanding of quark behaviors and hadronic structures. This research isn’t only about answering fundamental questions of particle physics; it’s about laying the groundwork for future discoveries that could reshape our comprehension of the universe.