Propagation of QCD Color through Strongly Interacting Systems
QCD, the theory of the strong interaction, has three conserved color charges, just as QED has one conserved electric charge. Due to the QCD property of confinement, these charges are not visible in isolation, but rather are only found in color-neutral combinations, the mesons and baryons. However, color can be briefly liberated in high-energy scattering interactions, and can travel relatively long distances before coalescing back into color-neutral systems. By inducing the high-energy scattering to occur inside atomic nuclei, which are spatially extended systems, one can infer properties of the propagation of color through these systems, and can in some cases extrapolate to the behavior in the vacuum.
The approach taken is to measure the modifications of observables in these spatially extended systems compared to smaller systems, such as the proton or deuteron, and to then use these modifications to understand the space-time features of color propagation. New technologies over the past two decades have enabled, for the first time, sub-femtometer-scale measurements on atomic nuclei in experiments with fully identified hadrons. The first measurements have come from the HERMES experiment at DESY and the CLAS experiment at Jefferson Lab. Further, over the past eight years, the heavy ion program at the Large Hadron Collider has probed this physics in the hottest and densest systems in the universe. The unfolding story of the study of these topics will be explained in the colloquium. In addition, the prospects for extending these studies in the future will be discussed, using the upgraded CLAS12 detector, the upgraded ATLAS detector at the LHC, and ultimately with the future Electron Ion Collider.