An experimental test of the GDH sum rule

The GDH sum rule for the proton has been tested via measurements at MAMI (200-800 MeV) and at ELSA (0.6-2.9 GeV) and was found to be correct within the experimental uncertainties. Furthermore, in the photon energy range from 815 to 1825 MeV the helicity dependent total photoabsorption cross sections have been measured for the first time at ELSA on the deuteron. Circularly polarized tagged photons impinging on a longitudinally polarized LiD target have been used together with a highly efficient 4π detector system. From the measured energy range an experimental contribution to the GDH integral on the neutron of (33.9±5.5(stat)±4.5(syst)) μb is extracted. However, existing predictions from multipole analyses around 1 GeV are not compatible with the measured data.