Seeing Quarks in the Nucleon Through Wide Angle Compton Scattering

Dr. Jixie Zhang
Department of Physics
University of Virginia, Charlottesville, VA

Whether pQCD can describe exclusive reactions at medium energies remains an open question and an area of active study. Real Compton scattering (RCS) has the potential to provide insight to this unsettled issue.  In a pQCD description of RCS, the participation of three active quarks and two hard gluons is required. However the resulting predictions of pQCD for the RCS cross sections disagree with the experimental data. In contrast, calculations based on the handbag mechanism, involving a single quark coupled to the spectator through generalized parton distributions (GPDs), matches the cross section data well. A measurement of the longitudinal polarization transfer parameter KLL was found inconsistent with predictions of pQCD yet consistent with calculations within the handbag mechanism. Furthermore, Miller’s handbag approach, which includes quark and hadron helicity flip, contradicts pQCD where KLLALL, by finding that KLL ALL, where ALL is the initial state helicity correlation asymmetry.  The first ever measurement of ALL (E12-14-006) has been approved to run in Jefferson Lab’s Hall C and will be able to discriminate between the various models. E12-14-006 will utilize an untagged bremsstrahlung photon beam and the longitudinally polarized UVA/JLAB proton target.  After a brief introduction to the physics, the experiment will be described and the expected results presented.