Unveiling Meson and Nucleon Structure with Lattice QCD

Michigan State University, East Lansing MI

Investigators

Abstract

Understanding the fundamental structure of matter is one of the central goals of nuclear physics. Protons and neutrons, the particles that make up the nucleus of every atom, are themselves made of even smaller building blocks called quarks and gluons. These particles are governed by the laws of quantum chromodynamics (QCD), the theory that describes the strong force holding atomic nuclei together. Over the past six decades, scientists have made major strides in understanding how matter is built, but many mysteries remain. How do quarks and gluons generate most of the mass of particles like protons and neutrons? How do they contribute to the spin of these particles? And why does a pion, made with one fewer quark, have about ten times less mass? To answer these questions, major research facilities like the Jefferson Lab and the future Electron-Ion Collider conduct cutting-edge experiments to map the internal structure of protons, neutrons, and other particles. At the same time, advances in supercomputing allow scientists to simulate QCD directly, providing critical insights into aspects of the strong force that are difficult to measure experimentally. Lattice QCD (LQCD) is a powerful computational approach that enables direct simulations on supercomputers of QCD, the fundamental theory of the strong interaction, by discretizing spacetime into four-dimensional grids. This method allows the study of the properties of protons, neutrons, and mesons from first principles, starting directly from the underlying quarks and gluons, with fully controllable numerical and systematic uncertainties. Over the past decade, there has been remarkable progress in developing efficient algorithms for generating QCD gauge configurations and in advancing analysis techniques to extract physical information from LQCD data. In some areas, LQCD calculations have progressed to the point where they not only complement experimental measurements but also help guide them. This research focuses on two key directions. The first is delivering high-precision LQCD calculations of parton distribution functions (PDFs), including comprehensive control over lattice systematics: varying lattice spacings, volumes, and working directly at the physical pion mass. Integrating these precise LQCD results with global analyses of experimental data will significantly improve our understanding of the internal structure of hadrons. The second direction extends prior LQCD work to three-dimensional hadron structures using quasi- and pseudo-PDF methods. This includes studies of generalized parton distributions (GPDs) with an emphasis on exploring their skewness dependence, providing deeper insights into the spatial distributions of quarks inside hadrons. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.