The isospin structure of photoproduction of πη pairs from the nucleon in the threshold region

Photoproduction of πη -pairs from nucleons has been investigated from threshold up to incident photon energies of ≈ 1.4 GeV. The quasi-free reactions γp→pπ0η , γn→nπ0η , γp→nπ+η , and γn→pπ−η were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the γp→nπ+η reaction was measured for the first time). For the π0η final state coherent production via the γd→dπ0η reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost 4 π coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation σ(pπ0η)≈σ(nπ0η)≈2σ(pπ−η)≈2σ(nπ+η) as expected for a dominant contribution from a Δ⋆→ηΔ(1232)→πηN reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon–meson and π – η pairs. The experimental results are compared to the predictions from an isobar reaction model

Photoproduction of ω mesons off the proton

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

Data on Is and Ic in γ→p→pπ0π0 reveal cascade decays of N(1900) via N(1520)π

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

<math altimg="si1.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>Λ</mi></math> and <math altimg="si2.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>Σ</mi></mrow><mrow><mn>0</mn></mrow></msup></math> photoproduction with fine center-of-mass energy resolution

Measurements of <math altimg="si3.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mi>γ</mi><mi>p</mi><mo stretchy="false">→</mo><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><mi>Λ</mi></math> and <math altimg="si4.gif" xmlns="http://www.w3.org/1998/Math/MathML"><mi>γ</mi><mi>p</mi><mo stretchy="false">→</mo><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup><msup><mrow><mi>Σ</mi></mrow><mrow><mn>0</mn></mrow></msup></math> cross-sections have been obtained with the photon tagging facility and the Crystal Ball calorimeter at MAMI-C. The measurement uses a novel <math altimg="si5.gif" xmlns="http://www.w3.org/1998/Math/MathML"><msup><mrow><mi>K</mi></mrow><mrow><mo>+</mo></mrow></msup></math> meson identification technique in which the weak decay products are characterized using the energy and timing characteristics of the energy deposit in the calorimeter, a method that has the potential to be applied at many other facilities. The fine center-of-mass energy ( W ) resolution and statistical accuracy of the new data results in a significant impact on partial wave analyses aiming to better establish the excitation spectrum of the nucleon. The new analyses disfavor a strong role for quark–diquark dynamics in the nucleon