Reaction mechanism of quasi-free knockout processes in exotic RI beam era

Abstract The quasi-free nucleon knockout reaction has revealed the single-particle nature of nuclei. Thanks to the advances in experimental techniques and reaction theory, various new aspects of nuclei are being discovered using knockout reactions. In this article, we review the basic concept of the quasi-free knockout reaction, and recent achievements mainly at RIKEN using the liquid hydrogen target and detector system. We also present our new findings on the low-energy nucleon knockout reaction and the α knockout reaction. The combination of the (microscopic) structure theory, reaction theory and experiments will be the key to a complete understanding of the α formation. Although the α clustering phenomena have been studied intensively in the light mass region, its existence in the wide mass region of the nuclear chart is still unknown. This is within the scope of the ONOKORO project, as well as noble clusters, e.g., d, t, and 3He. The implementation of two-nucleon (or more) correlations in the reaction theory is essential to connect the properties of such clusters and the reaction observables. A new framework is introduced for this purpose, which will also be applicable to the two-nucleon knockout reactions, e.g., (p, 3p), (p, 2pn), and (p, p2n).