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

Abstract

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

accepted for publication in Phys. Lett. B