Photoproduction of mesons off nuclei

Photoproduction of mesons off nuclei, in particular the deuteron, helium-, and lithium-isotopes, but also heavier ones up to lead, has been studied at the tagged photon beams of the electron accelerators ELSA in Bonn and MAMI in Mainz with the Crystal Barrel/TAPS and Crystal Ball/TAPS experiments. The aim of these measurements was twofold: the study of meson production reactions off quasi-free neutrons and the study of meson–nucleus interactions. The measurements of quasi-free reactions off the neutron include the observation of a narrow structure, of as yet unknown nature, in the excitation function of η production off the neutron. These experiments continue now with the measurement of single and double polarization observables. The main topics for the meson–nucleus interactions were the search for η -mesic nuclei and the investigation of nuclear double-pion production reactions in view of possible in-medium modifications of the σ -meson.