Search of η-nucleus bound state formation in recoil free transfer reactions

We have performed an experiment to search for the bound state of η meson in nuclei. We have measured the reaction p + ²⁷Al → ³He + p + π^- + X at recoil-free kinematics. An η meson possibly produced in this reaction would be thus almost at rest in the laboratory system and could therefore be bound with high probability, if bound nuclear η states exist. The decay of such a state through the N*(1535) resonance would lead to a proton-π^- pair emitted in opposite directions. For these conditions we find some indication of such a bound state. An upper limit of ≈ 0.5 nb for the formation cross section is found. (author)