Correlations in Nuclei: Recent Progress on an Old Problem

The two preeminent features of the nucleon-nucleon ($N$$N$) interaction are its short-range repulsion and intermediate- to long-range tensor character. These induce strong spatial-spin-isospin $N$$N$ correlations, which leave their imprint on the structure of ground- and excited-state wave functions. In the present talk I will review how these features influence a variety of nuclear properties---from energy spectra of low-lying states to two-nucleon density distributions to nuclear response functions---as well as the experimental evidence in support of their presence. In particular, I will show [R.\ Schiavilla, R.B.\ Wiringa, S.C.\ Pieper, and J.Carlson, Phys.\ Rev.\ Lett.\ {\bf 98}, 132501 (2007)] how tensor correlations impact the momentum distribution of $n$$p$ pairs in the ground state of nuclei and make it orders of magnitude larger than that of $p$$p$ pairs for values of the relative momentum in the range (300--600) MeV/c and vanishing total momentum. This order-of-magnitude difference is seen in all nuclei considered, and has a universal character originating from the tensor components present in any realistic $N$$N$ interaction. It should be easily observable in two-nucleon knock-out processes. Indeed, a preliminary analysis of $(e,e^\prime np)$ and $(e,e^\prime pp)$ reactions in $^{12}$C finds [R.\ Subedi {\it et al.}, in preparation] that the $p$$p$ cross section is suppressed relative to the $n$$p$ by a factor $\simeq 10$ in kinematics close to back-to-back emission of the two nucleons.