Exploring the nuclear landscape  Nuclear Structure under Extreme  Introduction

Nuclei far from Stability

The study of exotic nuclei, far from the valley of beta stability, presents an important challenge for nuclear physics. Nuclear models have to be radically improved in order to accommodate the wealth of new phenomena that have been unveiled by the continuing experimental work. Prominent milestones are: the discovery of the heaviest chemical elements; the discovery of nuclear halos in the lightest neutron-rich nuclei, immediately followed by a widespread theoretical effort that has considerably advanced our understanding of this phenomena; the experimental mapping of the nuclear shell structure far from stability, reaching in some cases the neutron or proton drip lines. Although we are still far from having a unified theoretical description valid far from stability, important progress has been made. New clues to the isospin dependence of the spin-orbit field have been given by the relativistic mean field calculations, and the study of the role of the proton-neutron pairing is being actively pursued in N=Z proton rich nuclei and this may provide new insight into the properties of the mean field of nuclei far from stability. The most detailed description of nuclear structure is given by the shell model and new techniques such as the Shell Model Monte Carlo Method may greatly help in dealing with the heavier nuclei. In this context, there have been improvements in the use of interactions obtained from nucleon-nucleon potentials that fit the N-N phase shifts. There are some aspects specific to the theoretical description of drip line nuclei, for example how to deal with resonant quasi-bound states. This calls for new developments regarding the treatment of the continuum and the correlations -- particularly pairing -- on the same footing, including few-body dynamics. Another example is the possibility of enhanced charge symmetry breaking due to Coulomb effects in quasi-bound orbits. The low-energy behaviour of giant resonances in drip line nuclei has also raised a lot of interest recently, be it soft dipole ``pigmy'' resonances or threshold peaks of the dipole and quadrupole channels. Many new features of nuclear structure have been found -- or are predicted to show up -- in the realm of exotic nuclei, a few of which are singled out in the following.

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Halos: Outstanding structural drip-line phenomena with extreme clusterisation into an ordinary core nucleus and a veil of halo nucleons - forming exceptionally dilute neutron matter. The origin is only partly understood, but a prerequisite is low angular momentum for the halo particles and few-body dynamics such as in Borromean nuclei characterised by pairwise constituents with no bound states. In the limit of vanishing binding extremely large halos may occur.

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Skins: For a large neutron excess, the bulk of the neutron density is predicted to extend beyond the proton density creating a sort of ``neutron skin''. Similar effects may appear for a large proton excess. There are many suggestions how these skins might change nuclear properties. One could imagine different deformations of the neutron and proton distributions etc.; first experimental observations and theoretical work hint into this direction. It provides the opportunity to study the behaviour of abnormal nuclear matter with very large T_z. This could be helpful for improving the reliability of calculations of the properties of neutron stars.

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Shell stabilisation: It has been verified that the heaviest elements owe their existence to the N=162 neutron deformed shell closure. These results strongly indicate that even heavier elements are shell stabilised

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Vanishing of shell closures: Theory and experiment are now indicating that shell closures may change far from stability. A well-known example is the disappearance of N=20 as a neutron magic number in the neon, magnesium and sodium isotopes. But the physical mechanism which is responsible for this disappearance could have other consequences for nuclei of increasing instability such as reduction of the shell gaps thus allowing many-particle, many-hole excitations to become more favoured in energy, leading to shape changes and shape coexistence

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Nuclei with N=Z: When protons and neutrons occupy the same orbitals, mutual reinforcement of the shell gaps may lead to a stabilisation of exotic shapes in the ground state. Here also the proton-neutron pairing interaction is exposed to view at its maximum. The study of isospin symmetry breaking, particularly for the heaviest mirror nuclei, is also of strong interest. As for medium heavy nuclei, the N=Z line coincides with the proton drip-line, proton decay from discrete excited states is possible, as recently observed in ⁵⁸Cu.

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Superdeformation and hyperdeformation: Far from the line of $\beta$-stability, medium heavy nuclei may exhibit large deformations, even at low excitation energies. Thus it has been shown that ⁷⁶Sr , ⁸⁰Zr and ¹⁰⁰Zr are amongst the most deformed nuclei known in their ground states. Recently a sequence of superdeformed states has been observed in ⁸⁰Sr and it is predicted that hyperdeformed states should exist in nuclei in this region as well.