Coherent photoproduction of pi^0- and eta-mesons off 7Li

Abstract

Coherent photoproduction of $\ddot{I}{€}^0$-mesons from threshold ($E_{th}\approx$ 136 MeV) throughout the $\hat{I}”$-resonance region and of $\hat{I}\cdot$-mesons close to the production threshold ($E_{th}\approx$ 570 MeV for $\hat{I}\cdot$) has been measured for ${}^7$Li 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$\ddot{I}€$ 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 $\hat{I}\cdot$-production had been only identified for the lightest nuclear systems (${}^2$H and ${}^3$He). For ${}^3$He 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 $\hat{I}\cdot$-production agree with a plane-wave approximation. Contrary to ${}^3$He, neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected form-factor dependence were observed.

accepted for publication in Eur. Phys. J. A