Baryons and baryoniums in the perspective of QCD sum rules

Following the experimental confirmation of tetraquark and pentaquark states, the search for hexaquark states has emerged as a new frontier in hadron physics. The recent observation of $X(1840)$ and $X(1880)$ by BESIII collaboration has provided evidence for the predicted $p\bar{p}$ bound states. Such baryon-antibaryon configurations, encompassing both bound states and resonances, are commonly referred to as baryoniums and are regarded as promising hexaquark candidates. In this article, we provide a comprehensive review of the investigations into baryonium states and their constituents, i.e., baryons, within the framework of QCD sum rules. We delineate the fundamental calculation procedures of this method to facilitate its practical application and benchmark the theoretical predictions against alternative models as well as the latest experimental data.