Interaction of eta mesons with nuclei

TL;DR

This review summarizes decades of research on eta meson interactions with nuclei, focusing on experimental and theoretical efforts to understand eta-nucleon interactions and the potential existence of eta-mesic nuclei.

Contribution

It consolidates existing experimental and theoretical findings to clarify the eta-nucleon and eta-nucleus interactions and highlights uncertainties for future research directions.

Findings

Evidence suggests possible eta-mesic nuclei.

Uncertainties remain in eta-nucleon interaction parameters.

Experimental data partially support theoretical models.

Abstract

Back in the mid eighties, a new branch of investigation which was related to the interaction of eta mesons with nuclei came into existence. It started with the theoretical prediction of possible exotic states of eta mesons and nuclei bound by the strong interaction and later developed into an extensive experimental program to search for such unstable states as well as understand the underlying interaction via eta meson producing reactions. The vast literature of experimental as well as theoretical works which studied various aspects of eta producing reactions such as the $\pi ^+$ $n$ $\to \eta p$, $p d \to ^3$He $\eta$, $p \,^6$Li $\to ^7$Be $\eta$ and $\gamma ^3$He $\to \eta$ X, to name a few, had but one objective in mind: to understand the eta - nucleon ($\eta N$) and hence the $\eta$-nucleus interaction which could explain the production data and confirm the existence of some $\eta$-mesic nuclei. In spite of these efforts, there remain uncertainties in the knowledge of the $\eta N$ and hence the $\eta$-nucleus interaction. The present review is hence an attempt to bind together the findings in these works and draw some global and specific conclusions which can be useful for future explorations.