Study of medium effect in NN interaction by using (p,pn) reactions

Exclusive measurement of nucleon quasifree scattering is a direct tool to investigate how nucleon-nucleon ($NN$) interaction changes in the nuclear medium. We measured analyzing powers ($A_y$) for 1s$_{1/2}$-knockout ($p$,$pn$) reactions on light nuclei aiming at studying such modification. The experiment was performed at RCNP with a 392 MeV proton beam. It is found that the $^{12}$C($p$,$pn$) exclusive $A_y$ data are mostly consistent with impulse calculations based on the $p$-$n$ interaction in free space, in contrast to the ($p$,2$p$) case, where a strong suppression of $A_y$ is observed. This large imbalance between the ($p$,$pn$) and ($p$,2$p$) results is difficult to explain in conventional models.