Renewed interest in the physics of nuclei is fueled by experiments at rare isotope beam facilities, which open the door to new regions of exotic nuclei; by astrophysical observations and simulations of neutron stars and supernovae, which require controlled extrapolations of the equation of state of nucleonic matter in density, temperature, and proton fraction; and by studies of universal physics, which unite cold atom and dilute neutron physics. The projects funded by this award address this physics by combining effective field theory, which provides a systematic expansion of nuclear forces including the important three-body forces, and renormalization group methods, which decouple troublesome high-energy contributions from the low-energy parts we want to describe. The results are controlled calculations that can better exploit the steady improvements in computing power.
The training received by undergraduates, graduate students, and postdoctoral research associates in carrying out the proposed activities contributes directly to the building of a diverse scientific workforce. The mix of analytical and numerical computation employed is excellent preparation for both academic and industrial research. The emphasis on universal aspects of physical systems, fostered by effective field theory and renormalization group approaches, as well as the direct application of techniques to problems beyond the nuclear domain, leads to enhanced collaborations between disciplines. Outreach efforts will bring interactive science to a wide range of Ohio residents.
The strong interaction is the force that binds atomic nuclei. But this force is not naturally available in a form that is practical to use in calculations of the properties and reactions of nuclei, unlike the case for electromagnetism. In fact, it is not easily well-defined and there are many realistic nuclear potentials. The methods developed and applied in this project establish this force in a systematic expansion (using effective field theory or EFT) and then modify it for practical applications (using the similarity renormalization group or SRG). New calculations were made of light and medium-mass nuclei and of astrophysical systems (e.g., neutron stars). Other research focuses on electron scattering from light nuclei, the reaction mechanism and the investigation of short distance effects in these situations. The problems attacked in this project contribute to answering the central questions in nuclear physics as identified in the NSAC Long Range Plan. Specific contributions are summarized below. The EFT and RG methods for nuclear forces and additional techniques for finite basis corrections in many-body calculations that were developed in this project are being used by many groups in low-energy nuclear physics. The training received by students and postdoctoral research associates in carrying out the project has contributed directly to the building of a diverse scientific workforce. The mix of analytical and numerical computation our students and postdocs employed to solve complex problems has been excellent preparation for both academic and industrial research. Four graduate students and two postdoctoral research associates were trained during the project. Results have been disseminated through publications in refereed journals and through talks at conferences and workshops. A three-week course on "Nuclear forces and their impact on nuclear structure, reactions, and astrophysics" was developed as part of an initiative to train the next generation of young scientists in low-energy nuclear theory. Outreach efforts have brought interactive science to a wide range of Ohio residents. Specific accomplishments in this project include: * The SRG is used to soften interactions for ab initio nuclear structure calculations by decoupling low- and high-energy Hamiltonian matrix elements. This requires developing techniques to handle the contributions of forces between three nucleons (protons or neutrons), as opposed to just pairwise interactions. We made benchmark calculations with SRG-evolved interactions in light nuclei. * We derived corrections to the ground-state energies and radii of atomic nuclei that result from the limitations of finite harmonic oscillator expansions. * Previous work describing electrodisintegration of the deuteron at high (GeV) energies was extended to a wider kinematic region. * The first complete next-to-next-to-next-to-leading order (N3LO) calculation of the neutron matter energy in chiral EFT was made, with results for astrophysics: for the supernova equation of state, the symmetry energy and its density derivative, and for the structure of neutron stars. * We provided equations of state (EOS) for a collaboration with astrophysicists using neutron-star merger simulations. The results indicate how gravitational wave detection (e.g., at LIGO) can constrain the EOS and properties of neutron stars (masses and radii). * Recent observations of large mass neutron stars were combined with low-density EOS based on chiral EFT to provide uncertainty bands for the EOS and neutron star masses and radii. For use in astrophysical simulations, we provided detailed numerical tables for a representative set of equations of state. * A framework to evolve three-nucleon (3N) forces in a plane-wave basis with the SRG was applied to consistent interactions derived from chiral EFT, which were used to make the first fully consistent SRG calculations of neutron matter. * A new framework for computing the SRG evolution of three-nucleon forces (3NF) in hyperspherical momentum representation was developed. This method allows for a clean visualization of the evolution of the three-nucleon forces, which manifests the SRG decoupling pattern and low-momentum universality. * SRG flow equations were applied to separable inverse scattering NN potentials to identify and understand the nature of two-body universality. * The SRG flow equations can be used to formulate an ab initio method to solve the many-body problem, the so called In-Medium SRG. A large-scale IM-SRG code capable of handling arbitrary NN and 3N interactions was developed and applied in an extensive study of medium- to heavy-mass closed-shell nuclei based on free-space SRG-evolved interactions. * The IM-SRG was formulated for open-shell nuclei using a multi-reference formalism based on developments from quantum chemistry. The resulting multi-reference IM-SRG (MR-IM-SRG) was used to perform the first ab initio study of even oxygen isotopes with chiral NN and 3N Hamiltonians. * We applied a nucleon-nucleon scattering amplitude parametrization based on the Regge model to the calculation of electron scattering from light nuclei.
