Results from the N* Program at Jefferson Lab

We discuss the results on the fundamental degrees of freedom underlying the nucleon excitation spectrum and how they evolve as the resonance transitions are investigated with increasingly better space-time resolution of the electromagnetic probe. Improved photocouplings for a number of resonant states, those for the N(1720)P₁₃ being significantly changed, have been determined and entered into the 2008 edition of the RPP. Strong sensitivity to the N(1900)P₁₃ state, listed now as a 2-star state in the same edition of RPP, has been observed in KΛ and KΣ photoproduction. None of the earlier observations of a Θ⁺₅(1540) was confirmed in a series of three Jefferson Lab high statistics dedicated measurements, and stringent upper limits on production cross sections were placed in several channels. For the four lowest excited states, the Δ(1232)P₃₃, N(1440)P₁₁, N(1520)D₁₃, and N(1535)S₁₁, the transition amplitudes have been measured in a wide range in photon virtuality Q². The amplitudes for the Δ(1232) show the importance of the pion-cloud contribution and do not show any sign of approaching the pQCD regime for Q² < 7 GeV². For the Roper resonance, N(1440)P₁₁, the data provide strong evidence for this state as a predominantly radial excitation of the nucleon as a 3-quark ground state. For the N(1535)S₁₁, comparison of the results extracted from π and η photo- and electroproduction data allowed one to specify the branching ratios of this state to the πN and ηN channels; they entered into the 2010 edition of the RPP. Measured for the first time, the longitudinal transition amplitude for the N(1535)S₁₁ became a challenge for quark models and can be indicative of large meson-cloud contributions or alternative representations of this state. The N(1520)D₁₃ clearly shows the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q² > 0.5 GeV² confirming a long-standing prediction of the constituent quark model. The search for undiscovered but predicted states continues to be pursued with a vigorous experimental program. While recent data from Jefferson Lab and elsewhere provide intriguing hints of new states, final conclusions will have to wait for the results of the broad experimental effort currently underway with CLAS, and subsequent analyses involving the EBAC at Jefferson Lab.