Coherent photoproduction of π0- and η-mesons off 7Li

Coherent photoproduction of pi(0)-mesons from threshold (E-th approximate to 136 MeV) throughout the Delta-resonance region and of eta-mesons close to the production threshold (E-th approximate to 570 MeV for eta) has been measured for Li-7 nuclei. The experiment was performed using the tagged-photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4 pi solid-angle electromagnetic calorimeter. The reactions were identified by a combined invariant-mass and missing-energy analysis. A comparison of the pion data to plane-wave impulse modelling tests the nuclear mass form factor. So far coherent eta production had been only identified for the lightest nuclear systems (H-2 and He-3). For He-3 a large enhancement of the cross section above plane-wave approximations had been reported, indicating the formation of a quasi-bound state. The present Li data for eta production agree with a plane-wave approximation. Contrary to He-3, neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected form factor dependence were observed.