First measurement of the helicity asymmetry E in eta photoproduction on the proton

Results are presented for the first measurement of the double-polarization helicity asymmetry E for the eta photoproduction reaction gamma p -> eta p. Data were obtained using the FROzen Spin Target (FROST) with the CLAS spectrometer in Hall B at Jefferson Lab, covering a range of center-of-mass energy W from threshold to 2.15 GeV and a large range in center-of-mass polar angle. As an initial application of these data, the results have been incorporated into the Julich-Bonn model to examine the case for the existence of a narrow N* resonance between 1.66 and 1.70 GeV. The addition of these data to the world database results in marked changes in the predictions for the Eobservable from that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances. (C) 2016 Published by Elsevier B.V.