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

Abstract

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

Towards Precision Spectroscopy of Baryonic Resonances

“…There is a clear structure slightly below W = 1.7 GeV. However, in the analysis it was found that the structure can be explained as a conventional effect from the interference of a few multipoles, and no new narrow structure is needed for the description of the data [13].…”

“…4. The E observable in the reaction γp → ηp has been recently measured by the CLAS collaboration [13] for the first time ever. The different lines correspond to two fits to differently binned data.…”

“…In several occasions, the group has analyzed new experimental data that were then published together with the analyses [12,13]. The amplitude used for these analysis is based on a fieldtheoretical framework using chiral interactions.…”

Scrutinizing the evidence for N(1685)

“…The interpretation of the structure as narrow resonance was supported by further studies [13][14][15][16], the results reported in [17] were ambiguous. In a study of the helicity asymmetry of the reaction γp → ηp, no evidence for N (1685) was found [18].…”

“…The new data on E for the reaction γd → ηp(n) in [25] -with a spectator neutron -differ significantly from first BnGa fits which were performed before the data on double-polarization observables on γp → ηp on protons became available [18,26]. To explore this discrepancy, we included the new MAMI data for η production off protons (bound in deuterons) [25] in the fits.…”