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Physics Background
Modern nuclear physics explores nucleonic matter under extreme conditions that can be created in modern accelerator laboratories. Beams of exotic nuclei offer forefront research opportunities in nuclear structure physics and nuclear astrophysics. World-wide activity in the construction of radioactive nuclear beam (RNB) facilities bears witness to the interest in the physics that can be explored.
The complexities of nuclear many-body problems, in which the interactions between constituents are still not fully settled, have made nuclear physics a phenomena-driven field. Progress has been achieved by the close interplay of theory and experiment. Nuclei reveal a rich variety of phenomena and dynamics that are unique to the nuclear domain. Remarkable new discoveries have recently been made, such as self-organisation of inhomogeneous distributions of matter in halos, and the melting of shell gaps. RNB physics, driven by such discoveries, has added new elements to the nuclear paradigm. Having only explored part of the nuclear landscape, we can expect that new discoveries will be made in the light of the new developments in accelerators, detectors, and theoretical analyses.
Over the last decade, a theoretical framework of growing predictive power has been emerging. This will undoubtedly help us to answer a number of new questions. How deeply can we probe the spatial and structural characteristics of nuclei, including exotic nuclei? To what extent can few-body features and cluster properties be discriminated in exclusive/complete experiments? These are important questions since most theoretical “observables” in quantum physics are not directly accessible to experiment. This applies even to large-scale geometrical characteristics such as nuclear radii. Consequently, it is not clear that procedures and standard reaction theory, tested for stable nuclei, apply at the driplines. Dripline nuclei are extreme quantum systems, where very dilute nuclear matter can be probed. Nuclei in extreme isospin states also allow us to test basic nuclear symmetries.
The interplay between structure and reactions is now attracting increasing attention and the important content in increasingly more exclusive observables is a future challenge. Safe progress certainly requires a realistic treatment of the relevant degrees of freedom of the constituents. taking the spatial granularity – the cluster structure of the collision partners – into account. Promising studies of (transfer) reactions that ‘filter’ exotic structural features, such as di-neutron configurations, have already been initiated. The influence of the structure of the nearby continuum also makes reactions with loosely bound nuclei a tool for advancing fundamental reaction theory. Energies ranging from low to high are needed if ambitions about an understanding in terms of fundamental constituents, such as ab initio microscopic calculations of structure and reactions, are going to be fulfilled.
Experimental Tools
The new experimental facilities being planned and built will cover experimental reaction studies for a wide range of energies with exotic nuclei far off stability, with emphasis on nuclear structure and dynamics, and astrophysical aspects. The new projects will demand, with their versatile experimental arrangements, many different kinds of reaction mechanisms with radioactive beams. In the past decade some of these types of reactions were explored, giving us some guides as to how to proceed. New projects, with much higher beam intensity, resolution, efficiency, etc; will lead to experimental results of high accuracy and completeness; e.g. quasi-elastic scattering experiments in reverse kinematics detecting all outgoing reaction products including the recoil protons from the target. This requires the development of reaction theory to the same accuracy that is currently achieved for dedicated light-nucleus few-body dynamics.
Theory Challenges
Achieving the above goals of reaction theory will require a concerted and co-operative effort of a suitable range of European theory groups. This involves those working on both nuclear structure and reactions, and those investigating light nuclei with few-body methods, those investigating heavier nuclei with many-body techniques, shell-model and mean-field methods, and those examining the few-body peripheral properties of these heavier nuclei. In all cases, emphasis will be placed on good treatment of nucleon-nucleon correlations, on non-perturbative methods, and on deriving predictions (using details of the detector configurations) that can be directly compared with the experimental results. The aim of the reaction theories and calculations must be that discrepancies between theory and experiment will give information, not about approximations made, but about outstanding structural features of the nuclei under investigation. More specifically, they will elucidate peripheral properties of those nuclei, the interaction between the different constituents and the details of electromagnetic currents. Methods that have been developed successfully in few-body physics will be extended to provide precise calculations of hadronic and electromagnetic reactions.
In order to establish the connections between theory and experiment, it is essential to bring together theoretical and experimental partnerships, and to ensure that a common medium is provided. It is necessary to have a full understanding of experiments in order for the theorists to predict the correct observables, taking into account for example the full apertures and acceptances of the detectors. It is similarly necessary for the experimentalists to agree with the theorists concerning which reactions can be accurately modelled, and which correlated observables are the best discriminators between theories, and which give the best determination of what is so far unspecified in the theories.