Coherent photoproduction of η-mesons off 3He - search for η-mesic nuclei

Coherent photoproduction of η-mesons off 3He, i.e. the reaction γ3He→η3He, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accelerator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the η→γγ and η→3π0→6γ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong ηHe3 final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of π0-proton back-to-back emission in the γ3He→π0pX reaction revealed a very complicated structure of the background and could not support previous conclusions.