A description of the excitation spectrum of the nucleon is a key element for understanding the strong interactions at low energies, where perturbative methods (pQCD) cannot be applied. Constituent quark models have proved successful in describing the first excited (N*) states in each partial wave, but the number of predicted states at higher energy is much higher than has been observed experimentally. It has been proposed that one explanation could be strong di-quark correlations inside the nucleon, which reduce the symmetry. A modern approach to resolving the problem of the "missing" resonances is based on coupled-channels calculations, combining traditional pion data with weaker N* decay channels, using both proton and neutron targets, and measuring polarization observables in addition to cross sections. The latest N* experiment at Jefferson Lab is g13, which is running from October 2006 to June 2007. It uses linearly and circularly polarized photon beams and a deuterium target. Due to the exceptional statistics (30-40 billion triggers), the latter will allow for the first time a detailed investigation of rescattering effects. This is particularly promising since recent results from the CEBAF Large Acceptance Spectrometer Collaboration show that Lambdas photoproduced on a proton are fully polarized for all kinematics. Thus, a new window is opened for investigating hyperon-nucleon interactions.
Coffee and cookies before the presentation at 3:50 pm, and refreshments after the presentation will both be served in Room 128.