No abstract

Abstract. Photoproduction off protons of the pπ 0 η three-body final state was studied with the Crystal Barrel/TAPS detector at the electron stretcher accelerator ELSA at Bonn for incident energies from the π 0 η production threshold up to 2.5 GeV. Differential cross sections and the total cross section are presented. The use of linearly polarized photons gives access to the polarization observables Σ, Is and I c , the latter two characterize beam asymmetries in case of three-body final states. ∆(1232)η, N (1535)1/2 − π, and pa0(980) are the dominant isobars contributing to the reaction. The partial wave analysis confirms the existence of some nucleon and ∆ resonances for which so far only fair evidence was reported. A large number of decay modes of known nucleon and ∆ resonances is presented. It is shown that detailed investigations of decay branching ratios may provide a key to unravelling the structure of nucleon and ∆ resonances.

No abstract

No abstract

The first measurement is reported of the double-polarization observable G in the photoproduction of neutral pions off protons, covering the photon energy range from 620 to 1120 MeV and the full solid angle. G describes the correlation between the photon polarization plane and the scattering plane for protons polarized along the direction of the incoming photon. The observable is highly sensitive to contributions from baryon resonances. The new results are compared to the predictions from SAID, MAID, and BnGa partial wave analyses. In spite of the long-lasting efforts to understand γp→pπ(0) as the simplest photoproduction reaction, surprisingly large differences between the new data and the latest predictions are observed which are traced to different contributions of the N(1535) resonance with spin parity J(P)=1/2(-) and N(1520) with J(P)=3/2(-). In the third resonance region, where N(1680) with J(P)=5/2(+) production dominates, the new data are reasonably close to the predictions.