Observation of the pη′ Cusp in the New Precise Beam Asymmetry Σ Data for γp→<…

Data on the beam asymmetry $\mathrm{\ensuremath{\Sigma}}$ in the photoproduction of $\ensuremath{\eta}$ mesons off protons are reported for tagged photon energies from 1130 to 1790 MeV (mass range from $W=1748\text{ }\text{ }\mathrm{MeV}$ to $W=2045\text{ }\text{ }\mathrm{MeV}$). The data cover the full solid angle that allows for a precise moment analysis. For the first time, a strong cusp effect in a polarization observable has been observed that is an effect of a branch-point singularity at the $p{\ensuremath{\eta}}^{\ensuremath{'}}$ threshold [${E}_{\ensuremath{\gamma}}=1447\text{ }\text{ }\mathrm{MeV}$ ($W=1896\text{ }\text{ }\mathrm{MeV}$)]. The latest BnGa partial wave analysis includes the new beam asymmetry data and yields a strong indication for the $N(1895){\frac{1}{2}}^{\ensuremath{-}}$ nucleon resonance, demonstrating the importance of including all singularities for a correct determination of partial waves and resonance parameters.