Microscopic Nuclear Structure Theory

University Of Arizona, Tucson AZ

Investigators

Abstract

Tremendous progress has been made in the last ten years in calculating the properties of atomic nuclei microscopically starting with the free nucleon-nucleon (NN) interaction. One of the most successful theoretical approaches in this regard is the No Core Shell Model(NCSM). Instead of invoking the standard shell-model assumption, that most of the nucleons occupy an inert core, one treats all A nucleons as being active. In this way one is able to formulate a systematic procedure to derive e.ective interactions starting from realistic microscopic ones. This procedure makes use of the similar structures that two- and three-body clusters are expected to have in all nuclei. It was shown that these e.ective interactions speed up convergence tremendously and make converged solutions possible. It has now become apparent that present theoretical techniques are able to accurately calculate the properties of light nuclei using the NN interaction as input. Because the results show a considerable underbinding, it is clear that some extra ingredient is necessary, such as threenucleon interactions. Hence, the NCSM will be expanded to include the three-body cluster (or e.ective interaction) based not only on the two-body microscopic forces, but also on realistic three-nucleon (3N) bare interactions. It is important do this, because, for example, the spectra of light nuclei are expected to be sensitive to the spin structure of the 3N interaction. With these higher-body interactions one will also be able to perform NCSM calculations for heavier nuclei in smaller basis spaces and obtain better convergence and, hence, more accurate results. The development of this three-body cluster approach is an major step forward, as it will enable one to study the e.ects of current 3N interaction models and models based on chiral perturbation theory on the spectra. Consequently, it will become an important tool to probe these interactions. By developing a technique for computing the three-body cluster exactly, one can also investigate its structure, depending upon the number of nucleons in the nuclide being studied and upon the size of the model space. One of the goals of all nuclear structure calculations is the understanding of collective structures based on microscopic calculations. The structure of 12C is an interesting example. The .rst excited 0+ state is believed to have a 3a structure. Converged solutions for this problem are a challenge, but are within the reach of NCSM calculations. Once they can be obtained, one can study in detail the di.erence between the ground state and excited state structures. Another important aspect of the current investigations is the extension of the NCSM methods to arbitary operators. The NCSM is based on a unitary transformation of the cluster Hamiltonian. When one is able to extend this transformation to transition or current operators, it becomes possible to extract meaningful values for physical observables from the NCSM wave functions. The prediction of electroweak properties of light nuclei will be an important application of this new technique.