Search

In constituent quark models, a tensor interaction, mixing quark spins with their relative motion, is introduced to reproduce the observed baryon spectrum. This has a consequence completely analogous to the nuclear tensor force between the n and p in deuterium. A D state component is mixed into what would otherwise be a purely S-wave object. The D-wave component breaks spherical symmetry, resulting in a non-vanishing matrix element for the nucleon and a static quadrupole moment and deformation for its first excited state, the Δ resonance, at ∽325 MeV. The magnitude and sign of this D-state component are quite sensitive to the internal structure of the proton and have been of great interest in recent years [1].

The Laser Electron Gamma Source (LEGS)[1] has recently begun operations at the National Synchrotron Light Source (NSLS) of Brookhaven National Laboratory. At LEGS, laser light is Compton backscattered from the 2.5 GeV electrons circulating in the X-Ray storage ring of the NSLS. The backscattered photons have gamma ray energies, k < 320 MeV. The corresponding scattered electrons are detected in a tagging spectrometer which is incorporated into the storage ring. This tag provides a determination of the photon energy to ~ 5 MeV [2]. Since the laser light is polarized, and since the spin-flip amplitude in Compton backscattering is very small, the resulting polarization of the gamma-ray beam is very high.