Exotic nuclei and nuclear forces

I overview new aspects of the structure of exotic nuclei as compared to stable nuclei, focusing on several characteristic effects of nuclear forces. The shell structure of nuclei has been proposed by Mayer and Jensen, and has been considered to be kept valid basically for all nuclei, with well-known magic numbers, 2, 8, 20, 28, 50, .... Nuclear forces were shown, very recently, to change this paradigm. It will be presented that the evolution of shell structure occurs in various ways as more neutrons and/or protons are added, and I will present basic points of this shell evolution in terms of the monopole interaction of nuclear forces. I will discuss three types of nuclear forces. The first one is the tensor force. The tensor force is one of the most fundamental nuclear forces, but its first-order effect on the shell structure has been clarified only recently in studies on exotic nuclei. The tensor force can change the spin-orbit splitting depending on the occupation of specific orbits. This results in changes of the shell structure in many nuclei, and consequently some of Mayer-Jensen's magic numbers are lost and new ones emerge, in certain nuclei. This mechanism can be understood in an intuitive way, meaning that the effect is general and robust. The second type of nuclear forces is central force. I will show a general but unknown property of the central force in the shell-model Hamiltonian that can describe nuclear properties in a good agreement with experiment. I will then demonstrate how it can be incorporated into a simple model of the central force, and will discuss how this force works in the shell evolution. Actually, by combining this central force with the tensor force, one can understand and foresee how the same proton-neutron interaction drives the shell evolution, for examples such as Sn/Sb isotopes, N = 20 nuclei and Ni/Cu isotopes. The distribution of single-particle strength is discussed also in comparison to (e, e'p) experiment on Ca-48. The shell evolution affects shapes of nuclei through Jahn-Teller-type mechanism, and a very interesting example with exotic Si isotopes is discussed. The third type of nuclear force is a three-body force, which originates in the Delta particle excitation as proposed by Fujita and Miyazawa many years ago. This force is shown to produce a repulsive interaction between valence neutrons after averaging effects from the third nucleon in the core. The same three-body force is responsible for neutron stars. By including such effects of the three-body force, one can predict the correct drip line of oxygen isotopes, for instance. Thus, the landscape of atomic nuclei varies in going from stable to exotic nuclei due to particular nuclear forces, leading to a paradigm shift. This paper overviews some basic ideas and selected examples.