Studies of two-and three-nucleon interactions in nuclear reactions

The modification of the properties of mesons and nucleons in the nuclear medium is one of the most interesting topics in current nuclear physics. The reduction of hadron masses in this medium has been predicted as an effect of the partial restoration of chiral symmetry in nuclear matter, and the modification of the nucleon spinor in nuclear matter has also been discussed in the framework of a Dirac approach.  These modifications may result in variations of the nucleon–nucleon (NN) interaction with density.  It is anticipated that it will be possible to examine these in-medium modifications of the NN interaction through proton-induced knockout reactions [(p,2p) and (p,pn)], which are regarded as NN scatterings in a nuclear field.  We introduce a significant body of evidence suggesting the existence of medium effects. A recent experimental progress is the finding of the isospin dependence (difference between p-p and p-n) of the analyzing-power (A_y) suppression in nuclear medium. Note that the strongly interacting p-n pairs (p-n short-range (SR) and tensor correlations) which have been shown to be a universal aspect of high momentum nucleons in nuclei may also be responsible for the modification. Thus the two-nucleon (2N) and three-nucleon (3N) SR and tensor correlations will be also discussed in connection with the medium modifications of NN interactions.
 The talk also reviews studies of 3N forces (3NFs) in few-body systems. Direct comparison between the Nd scattering data and the Faddeev calculations clearly shows the importance of the 3NFs. Recently, there has been a theoretical advance in solving the 4N scattering problem such as p+³He. Thus it is interesting to investigate the evolution of 3NF effects in 4N systems. It should be noted that the total isospin channel of the 3NFs is limited to T=1/2 for the Nd scattering whereas the T=3/2 channel can be studied in p+³He scattering. These recent experimental progresses will be given.