AUTHOR=Khreptak Aleksander , Skurzok Magdalena , Moskal Paweł 

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

JOURNAL=Frontiers in Physics

VOLUME=11

YEAR=2023

URL=https://www.frontiersin.org/journals/physics/articles/10.3389/fphy.2023.1186457

DOI=10.3389/fphy.2023.1186457

ISSN=2296-424X

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