Double-polarization observable G in neutral-pion photoproduction off the   proton
  This paper reports on a measurement of the double-polarization observable G in $\pi^0$ photoproduction off the proton using the CBELSA/TAPS experiment at the ELSA accelerator in Bonn. The observable G is determined from reactions of linearly-polarized photons with longitudinally-polarized protons. The polarized photons are produced by bremsstrahlung off a properly oriented diamond radiator. A frozen spin butanol target provides the polarized protons. The data cover the photon energy range from 617 to 1325 MeV and a wide angular range. The experimental results for G are compared to predictions by the Bonn-Gatchina (BnGa), J\"ulich-Bonn (J\"uBo), MAID and SAID partial wave analyses. Implications of the new data for the pion photoproduction multipoles are discussed. 
A. Thiel, H. Eberhardt, M. Lang, F. Afzal, A.V. Anisovich, B. Bantes,   D. Bayadilov, R. Beck, M. Bichow, K.-T. Brinkmann, S...
nucl-ex