The reaction γ p → π γ'p measured with circularly polarised photons and the magnetic dipole *moment of the Δ+(1232)

The magnetic dipole moment of the Δ+(1232) resonance is a fundamental observable and provides a stringent test of baryons structure calculations. A pioneering measurement was performed in year 2002 and delivered a result of (2.7 ± 2.2(exp))μN. However, a higher precision is needed to allow a clear selection between different structure models. A new dedicated experiment using the Crystal Ball and TAPS detectors has been performed 2004/2005 at MAMI, offering a superior 4π-acceptance and particle identification. In parallel progress has also be made by theory groups in the development of model calculations which are needed to extract the dipole magnetic moment for this reaction. The reaction was experimentally studied with circularly polarised photons so that in addition to the largely improved statistical quality of the angular distributions also a first attempt to extract the polarisation observable Σcirc can be made. Preliminary results will be presented. (orig.)