Quasi-free photoproduction of η-mesons off 3He nuclei

Quasi-free photoproduction of η-mesons has been measured off nucleons bound in 3He nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost 4π covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The η-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the S11(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final-state interaction effects

Photoproduction of mesons off nuclei - the photonuclear programs at ELSA and MAMI

Recent results for the photoproduction of mesons from nuclei with tagged bremsstrahlung beams are summarized. The experiments have been done at the Mainz MAMI accelerator with the Crystal Ball/TAPS setup and at the Bonn ELSA accelerator with the Crystal Barrel/TAPS detector. Two main physics topics are covered. The electromagnetic excitation spectrum of the neutron has been studied with meson photoproduction reactions off quasi-free neutrons from light nuclei. Particularly interesting results have been obtained for eta-photoproduction, where the excitation function of the neutron shows a pronounced, narrow structure which is not observed for the proton. The interaction of mesons with nuclear matter and the in-medium properties of hadrons under various aspects have been studied with meson photoproduction from nuclei, covering a large mass range (from the deuteron to lead). Questions like the possible formation of eta-mesic nuclei and in-medium modifications of the sigma-meson have been addressed