Physics Colloquium - Imaging Protons and Neutrons in 3D

Protons and neutrons, collectively called nucleons, make up almost all visible matter, yet their internal dynamics remain a central open problem in nuclear physics. A key question is how the proton's spin arises from quarks and gluons and their motion and interactions. I will review how experiments access the most fundamental structure of matter using polarized lepton scattering at Jefferson Lab and polarized proton collisions at the Relativistic Heavy Ion Collider. I will highlight progress toward a three-dimensional picture of nucleon structure that combines spatial imaging with momentum distributions. Looking ahead, I will outline how the future Electron-Ion Collider, along with new detector technologies, including the Barrel Imaging Calorimeter being developed at Argonne, will provide much higher precision and broader kinematic reach for studies of nucleon structure.