Research in Strong-Interaction Theory

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The strong interaction is responsible for the binding of protons and neutrons into atomic nuclei. Improved quantitative understanding of how this happens is essential not only for fundamental nuclear research at US experimental facilities but for progress in astrophysics, for experiments on the nature of neutrinos, and for applications to energy and homeland security. An era of precision calculations of nuclear structure and reactions is underway, enabled in part by research results from previous NSF grants to the PIs. Tools and diagnostics that have been developed will be applied to confront a range of issues raised by this progress. These issues include improving the description of forces and other inputs for microscopic calculations of nuclei, applying new methods to assess the theoretical uncertainties, and improving the extraction of information from experiment that minimally depends on model assumptions. 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 thethe students and postdocs must employ is excellent preparation for both academic and industrial research, which is validated by the strong track record of past members of the group. Particular issues to be addressed by this project include robust uncertainty quantification, parameter estimation for effective field theory (EFT), consistent operators for EFT and renormalization group (RG) methods, understanding regulator artifacts, formulating many-body power counting, explicating the role of high-momentum/short-distance physics (including short-range correlations), and extracting process-independent quantities from experiment. Projects will apply to inter-nucleon interactions, few-body and many-body systems, and electroweak probes. Specific tools to be used include Bayesian statistical methods for EFT, basis extrapolation methods, analysis of many-body contributions, contact (pionless) EFT, and similarity RG (SRG) evolution. These tools provide interconnections in attacking issues and extending applications; they will continue to be developed and will be made widely available. The target applications include Bayesian parameter estimation and model selection for hadronic and nuclear problems, new SRG generators to address the growth of many-body operators, consistent knock-out reactions for few-body systems, and the use of pionless EFT to study operator evolution and assess perturbative versus non-perturbative approaches. These all contribute to the goal of model independent, microscopic calculations of nuclei. The projects will impact forefront problems in low-energy nuclear physics such as the physics of nuclei far from stability, which is relevant for astrophysics and the upcoming Facility for Rare Isotope Beams (FRIB), and at Jefferson Laboratory.