Measurement of the double polarization observable G and beam asymmetry Σ in pion photoproduction

The excitation spectrum of baryons consists of many resonances which contribute selectively to distinct decay channels. To obtain information about the contributing resonances, Partial Wave Analyses (PWA) are performed to identify the resonances and characterize their properties. For an unambiguous partial wave analysis solution, the measurement of several well chosen single and double polarization observables is needed in different decay channels. The polarization observables are measured at the A2 experiment at MAMI in Mainz, using polarized photons and polarized nucleons. The setup covers nearly 4π of the solid angle and has a high detection efficiency for neutral and charged final states. It is therefore perfectly suited for the detection of pions in the final state. The preliminary results for the double polarization observable G and the beam asymmetry Σ in pion photoproduction are presented. They have been determined in an energy range E_γ=200-800 MeV, using a linearly polarized photon beam in combination with a longitudinally polarized butanol target. Therefore, it is the first measurement of the double polarization observable G below 500 MeV.