Mapping the Structure of Hadrons with Lattice QCD

Michigan State University, East Lansing MI

Investigators

Abstract

Understanding the innermost structure of visible matter is an important research area in nuclear physics. The basic building blocks of the atomic nucleus, protons and neutrons, are composed of elementary quark and gluon constituents governed by the theory of the strong interaction, quantum chromodynamics (QCD). Our knowledge has advanced greatly over the last six decades but many puzzles remain: How do these quarks and gluons make up the masses of composite particles? How do they contribute to their spin? How can a pion, composed of one fewer quark, have mass roughly 10 times lower? Many experimental facilities are also racing to answer these questions, such as the Jefferson Lab accelerator 12-GeV program and the planned Electron-Ion Collider. These experiments will advance the tomography of nucleons and other particles. At the same time, high-performance computing allows the direct simulation of QCD directly, providing information about aspects that are hard to access via experiments. Lattice QCD (LQCD) is a theoretical method to directly simulate QCD on four-dimensional spacetime lattices using supercomputers. Using this technique, the properties of hadronic systems can be studied from first principles (that is, from the underlying quark and gluon degrees of freedom) with fully controllable numerical and systematic uncertainties. In the past decade, there has been significant progress in the development of efficient algorithms for generating ensembles of QCD configurations and tools for extracting relevant information from LQCD correlation functions. In some cases, LQCD calculations have reached a level where they not only complement, but also guide experimental programs. Two main research directions are being pursued. The first is to provide precision LQCD calculations of nucleon couplings and moments with full studies of lattice systematics, including multiple lattice spacings and volumes, directly at physical pion mass. Combining precision LQCD inputs with ongoing and upcoming precision low-energy experiments can help set limits for new physics. The second direction is to investigate non-singlet hadron structure using quasi- and pseudo-PDF methods. Focus is on one of the biggest theoretical uncertainties in most global fits, the assumption of strange-antistrange symmetry and the gluon structure of nucleons and mesons. 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.