Experimental verification of the GDH sum rule

Sum rules involving the spin structure of the nucleon like those due to Bjorken, Ellis and Jaffe and the one due to Gerasimov, Drell and Hearn offer the opportunity to study the structure of strong interactions. At long distance scales in the confinement regime the Gerasimov-Drell-Hearn (GDH) Sum Rule connects static properties of the nucleon like the anomalous magnetic moment K and the nucleon mass M, with the spin dependent absorption of real photons with total cross sections sigma(3/2) and sigma(1/2): integral(0)(infinity) dv/v(sigma(3/2)(v) - sigma(1/2)(v)) = 2pi(2)alpha/m(2 .) kappa(2) Here 3/2 and 1/2 identify relative spin orientation of the photon and the nucleon parallel or anti-parallel respectively in the nucleon rest frame; a denotes the fine-structure constant and v the energy of the photon. Hence the full spin-dependent excitation spectrum of the nucleon is related to its static properties. The sum rule has not been investigated experimentally until recently. For the first time this fundamental sum rule is verified by the GDH-Collaboration with circularly polarized real photons and longitudinally polarized nucleons at the two accelerators ELSA and MAMI. The investigation of the response of the proton as well as of the neutron allows to perform an isospin decomposition. Data from the resonance region up to the onset of the Regge regime are shown. The "sum" on the left hand side of the GDH Sum Rule can be generalized to the case of virtual photons. This allows to establish a Q(2) dependency and to study the transition to the perturbative regime of QCD. This is the subject of several experiments e.g. at JLAB for the resonance region and of the HERMES experiment at DESY for higher Q(2). Moreover, this paper covers the status of theory concerning the GDH Sum Rule, the different experimental approaches and the results for the absorption of real and virtual photons will be reviewed.