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Measurements of Partial Channels of the GDH Sum Rule (Briscoe, Strakovsky) The Gerasimov-Drell-Hearn (GDH) sum rule relates the difference in the total hadronic photo-absorption cross sections for left- and right-handed circularly polarized photons interacting with longitudinally polarized nucleons to the anomalous magnetic moment of the nucleon squared divided by m2. The GDH sum rule provides an elegant connection between the nucleon structure functions obtained in high-energy lepton-scattering experiments and two static properties of the nucleon. Thus, it provides a bridge between perturbative and nonpertubative QCD. The fundamental interpretation of the GDH sum rule is that any particle which has a nonzero anomalous magnetic moment has internal structure and therefore an excitation spectrum. The detailed verification of the GDH sum rule shows which nucleon resonances play the most significant roles in this. There is a basic connection between the GDH sum rule which is valid for real photons and the famous Bjorken sum rule for virtual photons in the limit Q2 ® ¥ . The validity of the GDH sum rule for the proton has been satisfactorily established. In the case of the neutron, the experimental limits are still much too large; this is due substantially to the large uncertainty in the 2p photoproduction cross section, especially in gn ® popon in the photon energy range 500 to 1500 MeV. Most of the required experimental tools to remedy this, namely, high-quality circularly polarized tagged-photon beams and longitudinally polarized H2 and 3He targets are already available at MAMI up to 800 MeV and will be available up to 1.5 GeV in 2005. The Crystal Ball can provide the crucial 4p solid-angle multiphoton detector. An important byproduct of the 2po photoproduction data with polarized beams and targets is the help in the unraveling of the N* and D* resonance spectra, since the polarization data have enhanced sensitivity to small multipoles. |
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