Measurement of the helicity asymmetry E for the reaction $$ \gamma p\rightarrow \pi ^0 p$$: The CBELSA/TAPS Collaboration

A measurement of the double-polarization observable E for the reaction $$\gamma p\rightarrow \pi ^0 p$$ is reported. The data were taken with the CBELSA/TAPS experiment at the ELSA facility in Bonn using the Bonn frozen-spin butanol (C $$_4$$ H $$_9$$ OH) target, which provided longitudinally-polarized protons. Circularly-polarized photons were produced via bremsstrahlung of longitudinally-polarized electrons. The data cover the photon energy range from $$E_\gamma =600$$ to 2310 MeV and nearly the complete angular range. The results are compared to and have been included in recent partial wave analyses.