New data on $\vecγ \vec{p}\rightarrow ηp$ with polarized photons and protons and their implications for $N^* \to Nη$ decays
The polarization observables $T, E, P, H$, and $G$ in photoproduction of $η$ mesons off protons are measured for photon energies from threshold to $W=2400\,$MeV ($T$), 2280 MeV ($E$), 1620 MeV ($P, H$), or 1820 MeV ($G$), covering nearly the full solid angle. The data are compared to predictions from the SAID, MAID, JüBo, and BnGa partial-wave analyses. A refit within the BnGa approach including further data yields precise branching ratios for the $Nη$ decay of nucleon resonances. A $Nη$-branching ratio of $0.33\pm 0.04$ for $N(1650)1/2^-$ is found, which reduces the large and controversially discussed $Nη$-branching ratio difference of the two lowest mass $J^P=1/2^-$-resonances significantly.
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