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.

New data on the polarization observables T, P, and H for the reaction γp→pπ(0) are reported. The results are extracted from azimuthal asymmetries when a transversely polarized butanol target and a linearly polarized photon beam are used. The data were taken at the Bonn electron stretcher accelerator ELSA using the CBELSA/TAPS detector. These and earlier data are used to perform a truncated energy-independent partial wave analysis in sliced-energy bins. This energy-independent analysis is compared to the results from energy-dependent partial wave analyses.

The first measurement of the helicity dependence of the photoproduction cross section of single neutral pions off protons is reported for photon energies from 600 to 2300 MeV, covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, and BnGa partial wave analyses. Strikingly large differences between data and predictions are observed which are traced to differences in the helicity amplitudes of well known and established resonances. Precise values for the helicity amplitudes of several resonances are reported. PACS numbers:The spin structure of the proton has been of topical interest since the discovery that the quark spins constitute an unexpectedly small fraction of the proton spin [1]. Deep inelastic scattering experiments have revealed gluonic contributions to the proton spin to be consistent with zero, at least within the admittedly large errors [2]. At low energies, a different aspect of the proton spin structure is tested by a comparison of the helicity dependence of the total γp cross section integrated over all photon energies, ∞ 0 dE γ (σ 3/2 − σ 1/2 )/E γ , with the proton magnetic moment [3], a relation which is known as Gerasimov-Drell-Hearn (GDH) sum rule [4,5]. The subscripts denote the total helicity, h = 1/2 for photon and proton spin anti-aligned, h = 3/2 for both spins aligned. The GDH integral sums over all energies and all final states. A breakdown of the GDH integral into exclusive final states can provide a link between inclusive properties of the proton like its magnetic moment and the contributions of specific nucleon resonances to the GDH integral.In this letter, we report the first measurement of the double polarization observable(1) for the exclusive reaction γp → pπ 0 in the energy range from 600 to 2300 MeV, and compare the results with predictions of well established partial wave analyses (PWA) like SAID [6,7], MAID [8], and BnGa [9]. The double polarization observable G [10] -governing the correlation between linearly polarized photons and longitudinally polarized target protons -revealed remarkable differences in the predictions of the three partial wave analysis groups even in the well-studied 2 nd resonance region around E γ =750 MeV [11]. Here, we extend the covered energy regime to 2.3 GeV for the double polarization observable E. So far, data on E were published up to 780 MeV for a limited angular range [12,13]; further data from CLAS exist and were reported at a conference [14].The experiment was carried out using the tagged photon beam of the ELectron Stretcher Accelerator ELSA at Bonn [15]. Photons with circular polarization [16],were produced by scattering a 2.4 GeV beam of longitudinally polarized electrons with polarization P e − off a bremsstrahlung target. The electrons were deflected by a magnet into a tagging hodoscope which defines the energy of the bremsstrahlung photons. The electron polarization (P e − ≈ 0.60) was monitored in a Møller polarimeter [17]. The photon beam impinged on a butanol (C 4 H 9 OH) target, which was p...

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