Study of $\eta$ and ${\eta }^{\prime }$ photoproduction at MAMI

Abstract

The reactions $\gamma p\to \eta p$ and $\gamma p\to {\eta }^{\prime }p$ have been measured from their thresholds up to the center-of-mass energy $W=1.96$GeV with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections were obtained with unprecedented accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section and excitation functions for $\eta$ photoproduction at the energies in vicinity of the ${\eta }^{\prime }$ threshold, $W=1896$MeV ($E_{\gamma }=1447$MeV). This behavior is explained in a revised $\eta$MAID isobar model by a significant branching of the $N(1895)1/2^{-}$ nucleon resonance to both, $\eta p$ and ${\eta }^{\prime }p$, confirming the existence and constraining the properties of this poorly known state.