Abstract
The Gerasimov‐Drell‐Hearn (GDH) sum rule is a fundamental theorem that connects ground state properties of the nucleon with its helicity dependent total photoabsorption cross sections. For the first time, this basic theorem has been experimentally verified by the GDH collaboration with circularly polarized photons and longitudinally polarized nucleons at the tagged photon facilities of MAMI (Mainz) (Eγ ⩽ 800 MeV) and ELSA (Bonn). (800 MeV ⩽ Eγ ⩽ 800 MeV).In this presentation, the experimental approach will be presented together with the final results obtained for the proton and the first data obtained using a deuteron target. Data from the resonance region up to the onset of the Regge regime will be shown. Systematic uncertainties related to the evaluation of the GDH sum rule for the neutron will be discussed.The behavior of the helicity dependence of the single pion photoproduction channels for Eγ ⩽ 800 MeV, mainly in connection with the excitation of the D13 resonance, will be also presented.
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