Search for $\eta$-mesic nuclei in recoil-free transfer reaction

Abstract

We have studied the reaction $p+{^{27}Al}\to {^3{He}}+p+\pi^-+X$ at recoil-free kinematics. An $\eta$ 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 nuclear $\eta$ states exist. The decay of such a state through the $N^*(1535)$ resonance would lead to a proton-$\pi^-$ pair emitted in opposite directions. For these conditions we find some indication of such a bound state. An upper limit of $\approx$ 0.5 nb is found.