Helicity dependence of the $\gamma N$ interaction and the GDH sum rule

Abstract

First measurements of pion photoproduction using circularly polarized photons on longitudinally polarized protons were carried out at MAMI (Mainz) in the energy range E γ = 200–800 MeV. Results of the helicity dependence of the total inclusive photoabsorption cross-section and of the pion photoproduction channels will be presented. These data provide new input for multipole analyses and determine the main contribution to the Gerasimov-Drell-Hearn integral and the forward spin polarizability γ0.