Scientific Achievements since the 1989 Long Range Plan

Hadron structure is certainly not the sole province of the electromagnetic community. Hadron structure studies are an active area of research in high energy physics, particularly spectroscopic-type searches for states to fill out the expected roster of the mesons and baryons. Results have been obtained in experiments using probes such as radiative J/ decays and antiproton annihilation at rest. Specifically electromagnetic results in the area of hadron structure physics have been limited to the analyses of older data in recent years due to the lack of suitable machines and beam time. Effort has been concentrated on development of the programs of the nucleon structure studies at CEBAF [Bu92], as well as a many planned experiments using real or virtual photons to study strangeness production and the spectroscopy of the meson sector [CE94] [St94].

New insights into nucleon structure was gained from the analysis of inclusive electron scattering and of pion electroproduction data in the resonance region [St91]. Unseparated transition form factors for resonances in the and mass regions indicate scaling behavior at large momentum transfer, while such a behavior is not observed for the . The latter result is consistent with results of pion production analysis, which shows the E2/M1 ratio to be small even at GeV, while pQCD predicts this ratio to approach unity. Leading order perturbative QCD contributions may therefore still be relatively small for the at these momentum transfers. Analyses also showed that the helicity structure of the lower mass resonance transitions, especially of the , plays the dominant role for explaining the underlying dynamics of the Drell-Hearn-Gerasimov helicity sum rule and its connection with the deep inelastic spin sum rules.

The E2/M1 mixing ratio in the N to transition (EMR) is a particularly sensitive measure of spin-dependent quark-quark forces. In constituent quark models, both the nucleon and the are constructed predominantly from orbital angular momentum components. A quark- quark tensor interaction, mediated by one-gluon exchange, introduces small admixtures, which break spherical symmetry and results in ``deformed" nucleons and 's. Predictions for the E2 transitions between these states at are quite small in constituent models, EMR = -(0.5 +/- 0.3)%, but significantly larger in models that explicitly contain pion fields, eg. -(1.5 +/-0.5)%with Chiral bags or -(4.5 +/-1.5)%with Skyrmions. (The negative phase of the EMR would correspond to an oblate intrinsic shape.) The last five years have seen a record number of attempts at extracting this small EMR value from old (mostly unpolarized) data.

The effects of E2 transitions can be enhanced through polarization observables. A set of complete measurements of the , , and reactions with polarized photons has recently been completed at LEGS. The observed E2 are actually quite large, but most of this results from interferences with E2 components of the Born amplitudes, which are not particularly interesting. The E2 part associated with the excitation of the must be extracted by decomposing the amplitudes into resonant + background components. Predictions including only the background are several standard deviations per point away from the new polarization data. Preliminary analyses of the new LEGS results, while somewhat model dependent, are consistent with EMR = % [Sa94]. Evidently, the orbital angular momentum components in the N/ are much larger than expected in the constituent quark model.