Do quarks play explicit role in nuclear structure?

Starting with the quark model of nucleon structure in which valence quarks are strongly correlated within a nucleon, light nuclei are constructed by assuming the similar correlations of the quarks of neighboring nucleons. Application of the model to larger collections of nucleons reveals the emergence of the face-centered cubic (FCC) symmetry at a nuclear level where nucleons are arranged in alternating spin--isospin layers. The FCC model of nuclear structure, which is isomorphic to the shell model and, moreover, composes the features of the liquid drop and cluster models, has been proposed by N.Cook about 30 years ago. Binding of nucleons in stable nuclei are provided by quark loops which form three and four nucleon correlations. On a quark level the nuclear shell closures correspond to the octahedral or truncated tetrahedral symmetry. Thus all nuclei even with closed shells are non-spherically symmetric. The quark loop that can be identified with three nucleon force results in a 'pairing' effect. And namely quark loops leading to four nucleon correlations are responsible for the binding energy enhancement in even-even nuclei which are formed by virtual alpha-clusters. For medium and heavy nuclei the arrangement of nucleons is modified by Coulomb repulsion of protons. This effect together with quark/nucleon correlations leads to deviation from the shell model expectations. The model can predict the boundary of the maximal numbers of proton and neutron excess, i.e. proton and neutron drip lines. Moreover, the quark loops are responsible for formation of exotic (borromean) nuclei. These loops corresponding triangular nucleon configurations ³H and ³He are building blocks of both stable and borromean nuclei. In borromean nuclei possessing maximal deformation, these configurations are weakly bound. The proposed model provides predictions for specific nuclear configurations and their shapes. According to our approach, there is a depression of nuclear matter in the central part of ⁴He. The model predicts the maximally possible neutron excess in helium isotopes is 4, i.e. the last helium bound state is ⁸He, and a bound state of ¹⁰He does not exist. This document is composed of an abstract and the slides of the presentation. (author)