At the heart of matter lies a complex realm that defies our basic understanding of solidity. Contrary to the static image we often conjure when thinking about atoms, the nucleus—comprised of protons and neutrons—reveals a vibrant arena of activity. Inside these nucleons, hadrons interact through a dynamic interplay of fundamental particles called quarks and gluons. Together, they form what physicists refer to as partons. This intricate structure is not merely theoretical speculation; a dedicated team of physicists has embarked on an ambitious project to map and elucidate the interactions of these partons, shedding light on the very fabric of matter itself.
Such pioneering efforts are led by the HadStruc Collaboration, a collective based at the Thomas Jefferson National Accelerator Facility (Jefferson Lab) in the U.S. This collaboration comprises nuclear physicists deeply committed to constructing a mathematical framework that captures the nuances of parton interactions. Their significant findings were recently published in the Journal of High Energy Physics, thus contributing critical insights to the fundamental understanding of hadrons.
Joseph Karpie, a postdoctoral researcher from Jefferson Lab’s Center for Theoretical and Computational Physics, states that the HadStruc group consists of experts spanning various esteemed institutions, including William & Mary and Old Dominion University. This multidisciplinary approach fortifies their work, allowing for diverse expertise in theoretical and computational physics.
The collaborative effort primarily focuses on a sophisticated methodology known as lattice quantum chromodynamics (QCD). This mathematical framework enables researchers to calculate the internal construction of protons and other hadrons, arising from underlying quark-gluon behavior. Hervé Dutrieux, another member of the collaboration, highlights their innovative endeavor to conceptualize hadronic structure through a three-dimensional lens, an advancement that hinges on the use of generalized parton distributions (GPDs).
Unlike traditional one-dimensional parton distribution functions (PDFs) that limit interpretative scope, GPDs provide a richer narrative about the distribution of quarks and gluons within protons. This multidimensional approach is particularly insightful when addressing the origin of the proton’s spin, a long-standing enigma in particle physics.
The quest to understand the genesis of the proton’s spin is a pivotal focus within the HadStruc research agenda. Initial measurements in 1987 revealed a surprising fact: the quarks alone contribute to less than half of the proton’s total spin. The unaccounted spin appears attributable to the intricate motion of partons and their interaction dynamics. Dutrieux emphasizes the potential of GPDs as a promising tool to extract insights into this orbital angular momentum, bridging the gap between theoretical predictions and experimental outcomes.
Moreover, exploring the concept of the energy momentum tensor is another critical aspect of their research. This tensor gives a comprehensive picture of how energy and momentum are distributed throughout a proton, reflecting its interactions with various forces including gravity. Presently, however, the team is focused on unraveling matter distribution—a foundational step toward broader inquiries into these complex dynamics.
To distill these intricate theoretical concepts into empirically verifiable models, the HadStruc Collaboration relies heavily on computational power. They executed a staggering 65,000 simulations using advanced supercomputers like Frontera and the Frontier supercomputer at Oak Ridge, collectively requiring millions of processing hours. This computational intensity is necessary to reasonably test their three-dimensional conceptualization of parton interactions.
Karpie remarks that this effort represents a significant milestone for the Department of Energy’s Quark-Gluon Tomography (QGT) initiative. The collaboration’s work is not merely theoretical; it holds promise for experimental validation in high-energy facilities worldwide, including ongoing investigations at Jefferson Lab. Techniques like deeply virtual Compton scattering (DVCS) and deeply virtual meson production (DVMP) are being utilized to empirically examine hadron structure through GPDs.
Looking Toward Future Discoveries
As the HadStruc team anticipates greater experimental capabilities with the upcoming Electron-Ion Collider (EIC) at Brookhaven National Laboratory, they are concurrently engaged in experiments that refine their theoretical models today. Continuous data collection at Jefferson Lab is augmenting the theoretical predictions, facilitating a comprehensive understanding that traverses the experimental-theoretical divide.
Karpie’s ambition extends beyond merely reacting to experimental outputs; he desires to foresee developments in QCD research, transitioning the field from a reactive to a predictive stance. This goal aligns with the overarching vision of advancing our comprehension of particle physics and the constituents of matter that compose the universe.
The efforts by the HadStruc Collaboration exemplify the synergy between theory and experiment in the realm of nuclear physics. As they endeavor to decode the intricate structure and dynamics of partons, the potential for groundbreaking discoveries looms large on the horizon. Continued advancements in computational methodologies and experimental techniques may someday illuminate the mysteries of how matter behaves at its most fundamental level.