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 b
aryon 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.
The photoproduction of $eta^{}$-mesons from different nuclei has been measured using the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. Recent results on the in-medium properties of the $eta^{}$-meson, derived from
the transparency ratio measurements, are presented. The absorption of the $eta^{}$-meson in nuclear matter is compared to the properties of the other mesons ($eta$ and $omega$).
Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector in the reaction $gamma pto ppi^0$ for photon energies between 0.85 and 2.50 GeV. The $pi^0$ mesons are observed in their dominant neutral decay mode: $pi^0togammagamma$.
For the first time, the differential cross sections cover the very forward region, $theta_{rm c.m.}<60^circ$. A partial-wave analysis of these data within the Bonn-Gatchina framework observes the high-mass resonances $G_{17}$(2190), $D_{13}$(2080), and $D_{15}$(2070).
Data on the photoproduction of $omega$ mesons on nuclei have been re-analyzed in a search for in-medium modifications. The data were taken with the Crystal Barrel(CB)/TAPS detector system at the ELSA accelerator facility in Bonn. First results from t
he analysis of the data set were published by D. Trnka et al. in Phys. Rev. Lett 94 (2005) 192303 cite{david}, claiming a lowering of the $omega$ mass in the nuclear medium by 14$%$ at normal nuclear matter density. The extracted $omega$ line shape was found to be sensitive to the background subtraction. For this reason a re-analysis of the same data set has been initiated and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the re-analysis and of the background determination are described. The $omega$ signal on the $Nb$ target, extracted in the re-analysis, does not show a deviation from the corresponding line shape on a $LH_2$ target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the $omega$ line shape to different in-medium modification scenarios is discussed.
Total and differential cross sections for $eta$ and $eta ^prime$ photoproduction off the proton have been determined with the CBELSA/TAPS detector for photon energies between 0.85 and 2.55 GeV. The $eta$ mesons are detected in their two neutral decay
modes, $etatogammagamma$ and $etato 3pi^0to 6gamma$, and for the first time, cover the full angular range in $rm cos theta_{cm}$ of the $eta$ meson. These new $eta$ photoproduction data are consistent with the earlier CB-ELSA results. The $eta ^prime$ mesons are observed in their neutral decay to $pi^0pi^0etato 6gamma$ and also extend the coverage in angular range.
