The polarization observables T,E,P,H, and G in photoproduction of η mesons off protons are measured for photon energies from threshold to W=2400 MeV (T), 2280 MeV (E), 1620 MeV (P,H), or 1820 MeV ...(G), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JüBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the Nη decay of nucleon resonances. A Nη-branching ratio of 0.33±0.04 for N(1650)1/2− is found, which reduces the large and controversially discussed Nη-branching ratio difference of the two lowest mass JP=1/2−-resonances significantly.
The differential cross sections and unpolarized spin-density matrix elements for the reaction γp→pω were measured using the CBELSA/TAPS experiment for initial photon energies ranging from the ...reaction threshold to 2.5 GeV. These observables were measured from the radiative decay of the ω meson, ω→π0γ. The cross sections cover the full angular range and show the full extent of the t-channel forward rise. The overall shape of the angular distributions in the differential cross sections and unpolarized spin-density matrix elements are in fair agreement with previous data. In addition, for the first time, a beam of linearly-polarized tagged photons in the energy range from 1150 MeV to 1650 MeV was used to extract polarized spin-density matrix elements.
These data were included in the Bonn–Gatchina partial wave analysis (PWA). The dominant contribution to ω photoproduction near threshold was found to be the 3/2+ partial wave, which is primarily due to the sub-threshold N(1720)3/2+ resonance. At higher energies, pomeron-exchange was found to dominate whereas π-exchange remained small. These t-channel contributions as well as further contributions from nucleon resonances were necessary to describe the entire dataset: the 1/2−, 3/2−, and 5/2+ partial waves were also found to contribute significantly.
The reaction γp→KS0Σ+ is studied in the photon energy range from threshold. Linearly polarised photon beams from coherent bremsstrahlung enabled the first measurement of photon beam asymmetries in ...this reaction up to Eγ=1650MeV. In addition, the recoil hyperon polarisation was determined through the asymmetry in the weak decay Σ+→pπ0 up to Eγ=2250MeV. The data are compared to partial wave analyses, and the possible impact on the interpretation of a recently observed prominent structure in the cross section near the K⁎ thresholds is discussed.
Photoproduction of two neutral pions off the proton is studied using linearly polarized photons, and the polarization observables Is and Ic are measured for the first time. These two observables are ...unique to multi-meson final states; they characterize correlations between the linear photon polarization and the direction of outgoing single particles in photoproduction of three-body final states. The Is and Ic distributions suggest that, in the 1.8 to 2.0 GeV mass region, the N(1520)3/2−π intermediate state is reached with reaction dynamics consistent with a dominant JP=3/2+ wave. These data are included in the Bonn–Gatchina (BnGa) partial wave analysis which is based on a large variety of data; the analysis confirms a significant contribution from the reaction chain γp→N(1900)3/2+→N(1520)3/2−π0→pπ0π0.
Data on the polarization observables T, P, and H for the reaction γp→pπ0 are reported. Compared to earlier data from other experiments, our data are more precise and extend the covered range in ...energy and angle substantially. The results were extracted from azimuthal asymmetries measured using a transversely polarized target and linearly polarized photons. The data were taken at the Bonn electron stretcher accelerator ELSA with the CBELSA/TAPS detector. Within the Bonn-Gatchina partial wave analysis, the new polarization data lead to a significant narrowing of the error band for the multipoles for neutral-pion photoproduction.
Properties of the Roper resonance, the first scalar excitation of the nucleon, are determined. Pole positions and residues of the P11 partial wave are studied in a combined analysis of pion- and ...photo-induced reactions. We find the Roper pole at {(1371±7)−i(92±10)} MeV and an elasticity of 0.61±0.03. The largest decay coupling is found for the Nσ (σ=(ππ)-S-wave). The analysis is based on new data on γp→pπ0π0 for photons in the energy range from the two-pion threshold to 820 MeV from TAPS at Mainz and from 0.4 to 1.3 GeV from Crystal Barrel at Bonn and includes further data from other experiments. The partial wave analysis excludes the possibility that the Roper resonance is split into two states with different partial decay widths.
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Precise data for quasi-free photoproduction of
η
-mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies ...up to 2.5GeV. The
η
-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to
η
-production off the free proton. No significant effects beyond the folding of the free cross-section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the
η
-nucleon pairs reconstructed from the final-state four-momenta of the particles. The total cross-section for quasi-free
η
-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant-mass spectrum of the
η
-neutron pairs. Position and width of the peak in the invariant-mass spectrum are
W
≈ 1665 MeV and FWHM
Γ
≈ 25 MeV. The data are compared to the results of different models.