Studies of hadronic structure using electromagnetic probes

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One of the central problems of modern physics research concerns our understanding of the building blocks of the atomic nucleus -- the protons, neutrons, and other particles (mesons) that bind them. Notable discoveries have indicated that these particles consist of yet more fundamental constituents, the quarks and gluons. While a very good theoretical framework (called Quantum Chromo-Dynamics, or QCD) is able to describe accurately how quarks and gluons interact at extremely high energies (or, equivalently, when the quarks are very close together), it has been very difficult to apply QCD to lower energy (longer distance) phenomena. One of the obstacles to our improved comprehension has been a lack of quality data, particularly for the Deep Exclusive Reactions which provide the clearest picture of the inner workings of QCD. These studies are difficult because they require continuous, high luminosity electron beams, and detectors with good particle identification and reproducible systematics. The Jefferson Lab (JLab) 12 GeV upgrade is intended to address these issues.