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

Coherent photoproduction of $\eta$-mesons off ${}^3$He, i.e. the reaction ${\gamma }^{3}He\to {\eta }^{3}He$, 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 $\eta \to \gamma \gamma$ and $\eta\rightarrow 3\pi^0\rightarrow 6\gamma$ 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 $\eta {}^{3}He$ final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of ${\pi }^0$-proton back-to-back emission in the $\gamma ^3{He}\rightarrow \pi^0 pX$ reaction revealed a very complicated structure of the background and could not support previous conclusions.