Experimental Verification of the GDH Sum Rule: A survey including the extension of the GDH integral to virtual photons

Abstract

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 κ and the nucleon mass m, with the spin dependent absorption of real photons with total cross sections σ3/2 and σ1/2: ∫ 0∞ dvv (σ3/2(v) − σ1/2(v)) = 2π2αm2 ⋅ κ2 (1) 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 de...