Sensitivity of γd → π0d Observables Near η-Threshold to the Intermediate ηNN Interaction and the Choice of Elementary Pion Production Amplitude

The role of intermediate eta NN interaction is studied in coherent pi(0)-photoproduction on the deuteron near eta-threshold using a three-body model with separable two-body interactions. This work is motivated by the measurements of the CLAS Collaboration at Jefferson laboratory, where a cusp-like structure in the energy dependence of the differential cross section has been observed at extremely backward pion angles. Results for differential and total cross sections are predicted and compared with available data. We found that the results for differential cross section differ significantly from those predicted by a simple model in which only the impulse approximation and two-body process contributions are considered. In addition, the sensitivity of the results for unpolarized cross sections and polarization observables to the elementary amplitude is investigated and considerable dependences are obtained. The results revealed that the intermediate eta NN three-body calculation and the choice of the elementary amplitude have a visible effect on the differential cross section and spin asymmetries. Compared to the experimental data from CLAS Collaboration for differential cross section, sizeable discrepancies are found.