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

TL;DR

This study investigates the potential existence of $ ext{eta}$-mesic nuclei by analyzing a specific nuclear reaction at recoil-free kinematics, aiming to detect bound states through decay signatures, and reports preliminary indications of such states.

Contribution

It presents the first experimental search for $ ext{eta}$-mesic nuclei using recoil-free transfer reactions and sets an upper limit on their production cross section.

Findings

Some indication of bound $ ext{eta}$-mesic states was observed.

An upper limit of approximately 0.5 nb on the production cross section was established.

The decay signature through the $N^*(1535)$ resonance was used to identify potential bound states.

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.