Ab Initio Description of Open-Shell Nuclei: Merging No-Core Shell Model and In-Medium Similarity Renormalization Group Approaches

The central task of ab initio nuclear structure theory is to solve the many-body Schrödinger equation for specific nuclei of interest. This equation can be cast as an eigenvalue problem that has to be solved either explicitly by diagonalizing extremely large matrices or implicitly by transforming the underlying nuclear Hamiltonian to a simplified structure.

A joint team of researchers from NSCL/FRIB and TU Darmstadt have proposed to merge two such explicit and implicit approaches, namely the No-Core Shell Model (NCSM) and the In-Medium Similarity Renormalization Group, in order to combine their advantages. The resulting method is the so-called In-Medium (IM) NCSM. In this approach, a continuous renormalization group transformation simplifies the structure of the nuclear Hamiltonian, and greatly accelerates the convergence of the NCSM diagonalization towards a controlled. Since the IM-NCSM is at its core a diagonalization method, it provides immediate and convenient access to the low-lying states of medium-mass nuclei, and the resulting wave functions can be used to confront a multitude of observables with experiment.

The IM-NCSM and its first applications are described in Phys. Rev. Lett. 118, 152503 (2017).