Proton Compton Scattering and Pion-Production with Polarized Photons

In analogy to the N-N tensor force in deuteron deformation, a tensor interaction, motivated by one-gluon exchange, between quarks introduces a D state component into the nucleon wave function. The experimental signature of such a quadrupole moment lies in the nucleon's first excited state, the Delta resonance which is photo-excited mainly by M1 photons. However, the D-wave component results in a small E2 transition strength which is quite sensitive to the internal structure of the proton. Since the tensor interaction mixes quark spins with their relative motion, polarization observables can be used to enhance the interference effects that are largely hidden in spin-averaged unpolarized measurements. The p(vec{gamma}, gamma p) and p(vec{gamma }, pi^circ) reactions were studied with linearly polarized gamma -rays between 210 and 330 MeV from the Laser Electron Gamma Source (LEGS) facility at Brookhaven National Laboratory. The two reaction channels were separated by detecting both scattered photons and recoil protons. The beam energy was tagged so that the energy dependent systematic errors were greatly reduced. Data for three sets of center of mass (CM) angle measurements, 60^circ, 90^ circ and 120^circ, are presented in terms of 12 energy bins (+/- 5 MeV). Results are at least in fair agreement comparing with selected model calculations. The measured polarization observables can be used for a new multipole analysis that will serve as a further constraint to the various theoretical models.