Helicity Structure of Pion Photoproduction and the GDH Sum Rule

First measurements of pion photoproduction using circularly polarized photons on longitudinally polarized protons were carried out at MAMI (Mainz) in the energy range Eγ =140-800 MeV. Preliminary results of the helicity dependence of the total inclusive photoabsorption cross section in the full energy range and of the partial reaction channels γp →nπ0 and γp→nπ+ in the Δ region are now available. These data provide new input for multipole analyses and determine the main contribution to the Gerasimov-Drell-Hearn (GDH) integral and the forward spin polarizability γ0. (author)