Measurement of double polarization observables in meson photoproduction   off the proton with the CBELSA/TAPS experiment
  One of the remaining problems within the standard model is to gain a good understanding of the low energy regime of QCD, where perturbative methods fail. One key towards a better understanding is baryon spectroscopy. Unfortunately, in the past most baryon spectroscopy data have been obtained only using $\pi$ N scattering. To gain access to resonances with small $\pi$ N partial width, photoproduction experiments, investigating various final states, provide essential information. In order to extract the contributing resonances, partial wave analyses need to be performed. Here, the complete experiment is required to unambiguously determine the contributing amplitudes. This involves the measurement of carefully chosen single and double polarization observables.   The Crystal Barrel/TAPS experiment with a longitudinally or transversely polarized target and an energy tagged, linearly or circularly polarized photon beam allows the measurement of a large set of polarization observables. Due to its good energy resolution, high detection efficiency for photons, and the nearly complete solid angle coverage, it is ideally suited for the measurement of the photoproduction of neutral mesons decaying into photons.   Preliminary results for the target asymmetry T, recoil polarization P and the double polarization observable H are discussed for $\pi^{0}$ and $\eta$ photoproduction off the proton. 
Jan Hartmann (for the CBELSA/TAPS collaboration)
nucl-ex