Photoproduction of multiple mesons off light nuclei

Photoproduction of π0π0-, π0η-pairs and π0π0π0-triplets off deuterium and off 3He have been observed at tagged photon beams of the Bonn ELSA accelerator with the combined Crystal Barrel - TAPS electromagnetic calorimeter (incident photon energies up to 2.5 GeV) and of the Mainz MAMI-C accelerator with the Crystal Ball - TAPS setup (incident photon energies up to 1.4 GeV). The mesons have been detected alone or in coincidence with recoil protons, neutrons and deuterons. This allows the measurement of meson production reactions off quasifree nucleons bound in the nucleus, as well as the coherent production off the nucleus. The comparison of quasifree proton reactions to free proton reactions can confirm or rule out possible nuclear effects on the extracted cross sections. Furthermore, the isospin composition of the electromagnetic excitation of a resonance can be deduced from the comparison of quasifree proton and neutron reactions. Moreover, for example, the measurement of the coherent photoproduction of π0π0-pairs off the deuteron, highlights the contributions of N* resonances since Δ* states are excluded by isospin selection. Angular, kinetic and mass distributions, total cross sections and, linear and circular asymmetries have been extracted and are presented.