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

TL;DR

This paper reports the first measurement of the helicity asymmetry E in eta photoproduction on protons, providing new data that influence theoretical models and enhance understanding of nucleon resonances.

Contribution

It presents the first experimental measurement of the helicity asymmetry E in eta photoproduction, using polarized targets and the CLAS detector, and integrates these results into theoretical models.

Findings

Data suggest possible existence of a narrow N* resonance between 1.66 and 1.70 GeV.

Inclusion of data significantly alters model predictions for the E observable.

Results improve understanding of nucleon resonance structures.

Abstract

Results are presented for the first measurement of the double-polarization helicity asymmetry E for the $\eta$ photoproduction reaction $\gamma p \rightarrow \eta p$. 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 J\"ulich 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 E observable using that model. Further comparison with several theoretical approaches indicates these data will significantly enhance our understanding of nucleon resonances.