The realm of particle physics often presents a perplexing dichotomy: while matter appears solid and continuous at a macroscopic scale, the interactions at the atomic level are anything but stable. Within the nucleus of atoms lie the hadrons, predominantly protons and neutrons, which are formed from an intricate dance of quarks and gluons. Collectively termed partons, these fundamental constituents engage in relentless interactions governed by the strong force, one of nature’s four fundamental interactions. An ambitious group of researchers, known as the HadStruc Collaboration, is forging ahead to elucidate the dynamic properties and spatial distributions of these partons, thus shedding light on how they construct hadrons.
Headquartered at the U.S. Department of Energy’s Thomas Jefferson National Accelerator Facility, the HadStruc Collaboration brings together a diverse team of nuclear physicists from leading institutions, such as William & Mary and Old Dominion University. The team, which includes theoretical and computational physicists, aims to develop a comprehensive mathematical framework that encapsulates the behavior of partons. Recent publications in the Journal of High Energy Physics underscore their persistent efforts to model the dynamic nature of hadronic structures. As Joseph Karpie, a postdoctoral researcher associated with the project, articulates, the collaboration not only measures existing theoretical models but seeks to expand the horizons of our understanding in quantum chromodynamics (QCD).
At the core of the hadronic structure are valence quarks—specifically, two up quarks and one down quark—that form the backbone of protons. These valence quarks are perpetually held together by gluons, the mediators of the strong force that constantly create and annihilate pairs of quark-antiquarks within the hadronic realm. This ceaseless fluctuation leads to a complex interplay of forces, ultimately giving rise to the proton’s properties. One pivotal discovery emerged in 1987, challenging preconceived notions about proton spin. Scientists found that quarks contribute less than half of the proton’s overall spin; instead, gluons and the spatial motion of partons play significant roles. This phenomenon invites further investigation into the contributions of gluon spin and orbital angular momentum, thereby posing rich questions that continue to challenge theoreticians.
Central to the HadStruc Collaboration’s work is the innovative mathematical framework known as generalized parton distributions (GPDs). This approach moves beyond the limitations of traditional one-dimensional parton distribution functions (PDFs) by offering a more nuanced perspective on the spatial arrangement of quarks and gluons. Hervé Dutrieux, a member of the collaboration, emphasizes that this three-dimensional understanding provides insights into pivotal questions about proton spin. The intricate relationships between spin and the underlying parton dynamics can now be scrutinized with greater fidelity, leading to a deeper comprehension of how the proton’s constituents interact.
Advancing such theoretical insights requires formidable computational resources. The HadStruc team has employed supercomputing facilities, such as Frontera at the Texas Advanced Computing Center and the Frontier supercomputer at Oak Ridge Leadership Computing Facility, to carry out approximately 65,000 simulations to validate their GPD framework. These extensive computational experiments involve simulating protons under various conditions and interactions, taking millions of hours to complete. Karpie underlines that these simulations serve not only as a test of their theoretical models but also provide insights that will push the boundaries of current experimental setups.
In tandem with their theoretical advancements, the HadStruc Collaboration is actively engaged in experimentation to test and refine their predictions. New experiments at Jefferson Lab are underway, examining hadronic structures and yielding data that complement computational models. The group anticipates that their findings will align with future investigations at the upcoming Electron-Ion Collider (EIC), aimed at delving deeper into hadronic structures beyond current experimental limits. The promise of these initiatives emphasizes the collaboration’s ambition to stay ahead in understanding quark and gluon interactions, a historic challenge in particle physics.
The exploration of hadronic structures through the lens of partons is far from over. As the HadStruc Collaboration continues to refine its methodologies and enhance computational models, it stands at the forefront of a transformative era in nuclear physics. The insights garnered from this work may unravel mysteries long embedded in the fabric of matter, revealing not just the structure of protons and neutrons but also illuminating fundamental questions surrounding the nature of existence itself. The pursuit of knowledge continues, driven by a curiosity as unfathomable as the particles under investigation.
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