Study of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>η</mml:mi></mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>η</mml:mi><mml:mo>′</mml:mo></mml:math> Photoproduction at MAMI

Abstract

The reactions $\ensuremath{\gamma}p\ensuremath{\rightarrow}\ensuremath{\eta}p$ and $\ensuremath{\gamma}p\ensuremath{\rightarrow}{\ensuremath{\eta}}^{\ensuremath{'}}p$ are measured from their thresholds up to the center-of-mass energy $W=1.96\text{ }\text{ }\mathrm{GeV}$ with the tagged-photon facilities at the Mainz Microtron, MAMI. Differential cross sections are obtained with unprecedented statistical accuracy, providing fine energy binning and full production-angle coverage. A strong cusp is observed in the total cross section for $\ensuremath{\eta}$ photoproduction at the energies in the vicinity of the ${\ensuremath{\eta}}^{\ensuremath{'}}$ threshold, $W=1896\text{ }\text{ }\mathrm{MeV}$ (${E}_{\ensuremath{\gamma}}=1447\text{ }\text{ }\mathrm{MeV}$). Within the framework of a revised $\ensuremath{\eta}\mathrm{MAID}$ isobar model, the cusp, in connection with a steep rise of the ${\ensuremath{\eta}}^{\ensuremath{'}}$ total cross section from its threshold, can only be explained by a strong coupling of the poorly known $N(1895)1/{2}^{\ensuremath{-}}$ state to both $\ensuremath{\eta}p$ and ${\ensuremath{\eta}}^{\ensuremath{'}}p$. Including the new high-accuracy results in the $\ensuremath{\eta}\mathrm{MAID}$ fit to available $\ensuremath{\eta}$ and ${\ensuremath{\eta}}^{\ensuremath{'}}$ photoproduction data allows the determination of the $N(1895)1/{2}^{\ensuremath{-}}$ properties.