This research project has the overall goal of increasing our knowledge of nuclear systems both finite and infinite. Particular emphasis is placed on the physics of nuclei with large neutron or proton excess with the intent to provide theoretical support and predictions for experiments at rare isotope facilities, including the new generation at RIKEN Tokyo, GSI Darmstadt, and NSCL/MSU East Lansing. Since most experiments at such facilities employ strongly interacting probes, an improved analysis of such reactions is proposed in order to extract new physics. The serious discrepancies between heavy-ion knockout and transfer reactions make this development urgent. The tool for this research project is the dispersive optical model, based on the Green's function method, which provides a bridge between nuclear reactions and nuclear structure information. Ab initio microscopic calculations of heavier and exotic nuclei are proposed based on the Faddeev random phase approximation (FRPA) formulation of the self-consistent Green's function method. This many-body approach appears to be one of the most promising strategies to deal with medium and heavy nuclei for the calculation of the ground state as well as spectroscopic data. The proposal also outlines concrete steps to clarify the phase diagram of nuclear matter around saturation density, as a function of temperature and nucleon asymmetry.
The research intends to raise the standard of the analysis of nuclear reactions and is intended to have a significant impact on the experimental program at facilities like the planned Facility for Rare Isotope Beams (FRIB). The ab initio FRPA method practiced in the group provides interdisciplinary links with other fields and involves international collaborations with institutions in England, Japan, Spain, and Belgium. The continued exposure of graduate students and young scientists all over the world to the many-body theory book of the PI fosters improved understanding of exciting new phenomena in nuclear physics in other disciplines. The mentoring of graduate assistants and post-doctoral associates in the process and ethic of scientific research is an integral part of the project. Their development toward independence as original and productive scientists is a central objective of the program and will continue to prepare them for industry or academic institutions.
Research during the recent grant period has applied the Green's method to the study of nuclei and the matter of neutron stars.This method was originally developed to describe fundamental processes between electrons and photons but can also be fruitfully applied to the study on nonrelativistic systems with identical particles like protons and neutrons in nuclei. In particular, the method has been implemented as the dispersive optical model (DOM) that uses experimental data like elastic scattering cross sections to constrain the potential that nucleons in the nucleus experience. In other words, these data can be used to understand how nucleons behave in nuclei. We have extended the DOM such that also experimental properties of ground states of nuclei can be employed to constrain the properties of nucleons in the nucleus. The method has then been utilized to predict new properties of nuclei that have exotic combinations of neutrons and protons that have not been studied in the laboratory yet. These predictions suggest that when protons are the minority species, adding neutrons will change their properties substantially, i.e. they become more strongly entangled with their neutron environment. Contrary to this behavior the majority neutron properties appear to be rather insensitive to the addition of more neutrons. The ingredients of the DOM can also be employed to analyze and describe nuclear reactions in which a neutron is added or removed from the nucleus when the beam is a deuteron (the nucleus of deuterium) or a proton (and a deuteron is detected as the final product). Such an analysis has been performed in collaboration with scientists from Michigan State University and demonstrates that the DOM ingredients are superior to standard ingredients provided by global optical potentials. Furthermore, the outcomes of such an analysis yield results that are consistent with those obtained from analyzing the reaction where an electron beam is used to remove a proton from the nucleus and establishes the probability that the proton is removed from relevant (partially) occupied orbits. This consistency is critical because such electron beam experiments are not available for the study of exotic nuclei. Such nuclei are currently studied at facilities like the National Superconducting Cyclotron Laboratory at Michigan State University. New construction at that facility is pending which will make even more exotic nuclei accessible for laboratory study. These nuclei also occur in compact objects in the universe that are called neutron stars. The study of the properties of nucleons in these exotic nuclei can only be performed with these nuclei as beams and must involve targets also comprised of protons and neutrons. To extract unambiguous information it is therefore necessary to understand all data related to ground states as well as scattering data. Our recent progress in the description of these transfer reactions suggest that this will indeed be possible in the future. More detailed understanding of the DOM ingredients has also been gleaned by performing calculations that attempt to explain the properties of nucleons in the nucleus by starting from the interaction between pairs of nucleons in free space. Such realistic interactions require intense computational efforts to incorporate their properties in the behavior of nucleons. The most advanced application of the Green's function method includes effects that treat the short distance behavior of nucleons when the are close together as well as when they are far apart in the nucleus. When the potentials are calculated for different isotopes of Calcium, some of the DOM properties can be better understood and deficiencies can be repaired. One such deficiency is that the potential that describes the environment of a nucleon in the nucleus does not only depend on where the nucleon is but also where it is going. Such non-local features are currently implemented in new analyses of experimental data which will make it possible to describe details of the charge distribution of nuclei including those with exotic rations of neutrons to protons as well as understand the properties of very fast protons in the nucleus that have been studied at Jefferson Laboratory in Virginia.
