Double-polarization observables, η-meson and two-pion photoproduction

Photoexcitation of the nucleon yields structures in the total absorption cross section that indicate the existence of resonances. In order to explore resonance contributions, the total photoabsorption cross section does not deliver a satisfying set of information. Fixing degrees of freedom by selection of partial reaction channels achieved a more detailed set of information over the last years. Further defined conditions could be reached in the recent past by fixing the polarization orientation and type between photon beam and target. This allows to observe double-polarization observables of the differential photoabsorption cross section. This article gives an overview of the double-polarization observables G and E presents what can be learned from the polarization dependent photoproduction of the η- and ππ mesons (Authors)