Search for -mesic nuclei: a review of experimental and theoretical advances

In the mid-1980s, theoreticians predicted possible exotic states of an eta (eta) meson and a nucleus bound by the strong interaction, so-called mesic nucleus. This led to extensive experimental efforts aimed at discovering such unstable states as well as understanding the underlying interactions via eta meson production reactions. As experiments yielded new data, more and more sophisticated theoretical models were developed to understand the fundamental eta-nucleon and eta-nucleus interactions. After more than 35 years of research in this field, several signals have been identified as possible indications of hypothetical eta-mesic nuclei, but there is still no general agreement or clear confirmation of their existence. In the paper, we review the study of the eta-mesic nuclei with an emphasis on recent research activities.