Results from the GDH-Experiment at MAMI

A. Thomas

Institut für Kernphysik, Johannes Gutenberg-Universität Mainz

Already in 1965 the Gerasimov-Drell-Hearn (GDH) sum rule, connecting the helicity dependent photoabsorption cross sections with the anomalous magnetic moment of the nucleon, was derived theoretically.

Some years ago the GDH collaboration has started an extended program at the electron accelerators MAMI in Mainz and ELSA in Bonn to measure the photoabsorption cross sections of circularly polarized photons on longitudinally polarized nucleons to determine for the first time a double polarization observable in a large kinematical range, which will provide new information about the helicity dependent excitation spectrum of the nucleon via an enhancement of small multipole amplitudes in interference terms.

In this talk our recently published data will be discussed and it will be shown how it is possible now, to check the behaviour of the GDH sum rule integral and to extract information about the nucleon resonances.

The experiment has been started using the polarized electron beam of the Mainz accelerator MAMI in the energy range 140 - 800 MeV and at the moment is continued at the Bonn accelerator ELSA up to an energy of approximately 3 GeV. We have finished the data taking period at MAMI for the proton in 1998 and in the outlook our plans to do the measurement on the polarized neutron in 2002 will be presented.