Helicity dependence of γp → nπ+. Results from the GDH experiment at MAMI

The helicity dependence of the total photoabsorption cross section and partial channels for polarized protons have been measured for incident photon energies from 120 to 800 MeV. We used the large acceptance detector DAPHNE with additional forward components in conjunction with a newly developed frozen spin target and the tagged polarized photon beam facility of the MAMI accelerator in Mainz (Glasgow-Mainz tagger). First preliminary results for the polarized total cross section of the γp → nπ+ channel and its contribution to the GDH integral and the forward spin polarizability are compared with partial wave analysis of HDT and SAID. (author)