Abstract: More than 99% of the mass of the visible matter resides in hadrons, which are bound states of quarks and gluons, collectively called partons. These are the fundamental constituents of quantum chromodynamics (QCD), the theory of the strong interactions. While QCD is a very elegant theory, it is highly nonlinear and cannot be solved analytically, posing severe limitations on our knowledge for the structure of the hadrons. Lattice QCD is a powerful first-principles formulation that enables the study of hadrons numerically, which is done by defining the continuous equations on a discrete Euclidean four-dimensional lattice.
Hadron structure is among the frontiers of nuclear and particle physics. The 2015 Nuclear Science Advisory Committee’s Long Range Plan for Nuclear Physics identified a future electron-ion collider (EIC) as the highest priority for new facility construction. Last year, the National Academies of Sciences, Engineering, and Medicine (NAS) released an assessment report thatstrongly endorses the science case for an EIC. The NAS report identified three high-priority science questions to understand hadron structure:
- How does the mass of the nucleon arise?
- How does the spin of the nucleon arise?
- What are the emergent properties of dense systems of gluons?
In this talk, I will discuss progress in lattice QCD related to aspects of the above questions, with a focus on the origin of the mass and the spin decomposition. I will show results for the proton, which provides an ideal system for studying QCD. I will discuss the strengths of lattice calculations, but also identify the challenges associated with elimination of systematic uncertainties.