Photoproduction of $\eta^\prime$ mesons off nuclei and their properties   in the nuclear medium
  We study the photoproduction of $\eta^\prime$ mesons from nuclei near the threshold within the collision model based on the nuclear spectral function. The model takes properly into account both primary photon-nucleon and secondary pion-nucleon $\eta^\prime$ production processes as well as the effect of the nuclear $\eta^\prime$ mean-field potential on these processes. We find that the secondary pion-induced reaction channel ${\pi}N \to {\eta^\prime}N$ plays an insignificant role in the $\eta^\prime$ photoproduction off nuclei. We calculate the transparency ratio for $eta^\prime$ mesons and compare it to the available experimental data. The comparison indicates an inelastic ${\eta^\prime}N$ cross section of the order of 6-10 mb. We show that the existing transparency ratio measurements do not allow one to distinguish between two adopted $\eta^\prime$ in-medium modification scenarios. Our studies also demonstrate that the momentum distribution and excitation function for $\eta^\prime$ production in ${\gamma}A$ reactions reveal some sensitivity to these scenarios, which means that such observables may be an important tool to get a valuable information on the $\eta^\prime$ in-medium properties. 
E. Ya. Paryev
nucl-th nucl-ex