NUCLEAR PHYSICS A ELSEVIER Nuclear Physics A623 (1997) 59c-70c The CEBAF N* Program Volker D. Burkert CEBAF/Jefferson Lab, 12000 Jefferson Avenue, Newport News, Virginia 23606, USA Physics topics relevant to the excitation of nucleon resonances are discussed. It is ar- gued that measurements of electromagnetic resonance transition amplitudes are sensitive to the QCD structure of the nucleon. The status and experimental schedule of the N* program at CEBAF are outlined. I. INTRODUCTION In electron scattering we may identify 3 distinct kinematical regions corresponding to different distance scales. At high energies and small distances the interaction involves ele- mentary quark and gluon fields, acting as quasifree particles. The interaction is described by perturbative QCD. At low energies and large distances, quarks and gluons appear in 'con- densed' form as nucleons and mesons, and the reaction is described by hadron theory. At intermediate distances, which we are concerned with, quarks and gluons are relevant, how- ever confinement plays a governing role, and quarks and gluons appear as constituent quarks and constituent glue, as for example in the flux tube model. 1 This picture, although quite successful in describing many aspects of hadron spectroscopy, is a model whose relationship to QCD remains unclear. Our hope must be that confrontation of model predictions with ac- curate data will show where this picture breaks down in non-trivial ways leading to improved models and to a better understartding of the nucleon structure in terms of its fundamental constituents. The goal of the N* program at CEBAF is to probe the internal structure of light quark baryons. This will be accomplished in various way: (1) by studying the resonance transition formfactors in a large Q2 range and for a large number of states. This will allow detailed tests of baryon structure models, (2) by searching for the so-called "missing resonances", states which are predicted in the symmetric quark model but have not been seen so far, and (3) by searching for gluonic excitations of the nucleon ("hybrid baryons"). In this talk I will use the constituent quark model2,3 (CQM) in its various implemen- tations (non-relativistic, relativized) as a guide. It provides physical insight and is aimed at a global description of both the mass spectrum as well as the structure of hadrons within a common framework. The model predicts a large number of resonant light quark (u,d,s) baryon states within the symmetry group SU(6) ® 0(3). The states fall into supermultiplets with fixed orbital angular momentum and energy excitation level. The mass degeneracy within one supermultiplet is broken by the color magnetic hyperfine coupling between the quark spins. I will be discussing transitions within the ground state (N-A), and from the ground state into states with excitation levels lhv and 2hi, only. These are of special interest since more accurate measurements can be made for the lower mass states. 0375-9474/97/$17.00 © 1997 -Elsevier ScienceB.V. All rights reserved. PII: S0375-9474(97)00423-5