E08-003: The γp→ π+n Single Charged Pion Photoproduction
Over the last several decades, the theory of Quantum Chromodynamics (QCD) has been established as the theory that describes how the nucleus is held together. In QCD, quarks and gluons are the fundamental constituents of protons and neutrons, which make up the nucleus, and the quarks interact via the exchange of gluons.
QCD is well tested at short distances; however, at larger distances, exact QCD calculations are prohibitively difficult. In this regime, effective theories that describe the strong force in terms of protons and neutrons exchanging mesons such as pions do a much better job. The interplay between these two pictures of the strong force represents one of the major issues in contemporary nuclear physics, and the physics connecting the two regimes remains unclear. The challenge is to study the transition region at intermediate energies and look for the onset of some experimentally accessible phenomena which are naturally predicted by QCD.
Single pion photoproduction lends itself very nicely for such studies, but previous measurements have been lacking in energy and angular coverage to provide a detailed picture of the transition region. Together with previous experiments both from Hall A and Hall B, the aforementioned transition region could be firmly established if nature indeed intends for such a transition to take place in the region to be accessed by this experiment. If not, these experiments pave the way for the future discovery of such a transition region at the energy-upgraded CEBAF.