The propagation of electrically charged particles is the foundation for many modern applications such as electron microscopy, free electron lasers, proton radiation therapy, and photon sources such as the Argonne APS. The main features of charged particle propagation through vacuum and through matter, and in the presence of external electromagnetic fields, were understood long ago. By contrast, the propagation of color-charged particles is just beginning to be understood. This process, which is governed by quantum chromodynamics (QCD), shares many features with the familiar electrodynamics process, such as energy loss by elastic scattering and bremsstrahlung. As with the electromagnetic process, it can be employed as a tool to probe extended systems such as the structure of nuclei and the properties of ultra-high density matter, and it can be used to measure new fundamental quantities in QCD, such as the lifetime of the quasi-free quark and formation times for hadrons. Unlike the electromagnetic process, however, the picture is complicated by quark confinement, which is dynamically enforced within the hadronization process accompanying the propagation of partons. A quantitative understanding of the related processes, including the role of the vacuum and of quantum coherence, is beginning to emerge, but there are many open questions. Parton propagation is a common feature linking all experiments that involve atomic nuclei in high energy scattering, thus uniting experiments at U.S. national laboratories including Fermilab, Brookhaven, and Jefferson Lab with measurements at CERN and DESY. I will review the physical processes involved at an elementary level, providing a brief survey of the most recent experimental data, and will outline the potential connections between diverse types of experiments. Following that, I will summarize some of the open questions and how they can be addressed by future measurements at Fermilab, Jefferson Lab, LHC, and the EIC.
Argonne Physics Division Colloquium Schedule