Heiko Hergert

Atomic nuclei are among nature’s most fascinating objects and a prime example of a strongly correlated quantum system. The interplay of complex nuclear interactions and many-body effects gives rise to a rich variety of nuclear phenomena, especially in exotic nuclei that are the focus of the experimental program at NSCL/FRIB. A reliable theoretical framework is required to advance our understanding of these phenomena, and to support both the planning and data analysis of future experiments.

The impact of this research program extends beyond nuclear structure and reactions research: Properties of finite nuclei are important ingredients for models of supernovae and neutron-star mergers, which are among the main nucleosynthesis sites in the cosmos. Furthermore, certain nuclei play key roles in neutrino detection or searches for physics beyond the Standard model, like the hypothetical neutrinoless double beta decay.

Ab initio (“from the beginning”, i.e., first-principles) nuclear many-body theory seeks to provide such a framework, by combining:

• Nuclear interactions from chiral effective field theory (EFT), which are  formulated  in  terms  of nucleons instead of quarks, but maintain a stringent link with Quantum Chromodynamics (QCD), the fundamental theory of the strong interaction in the Standard model,
• Renormalization group (RG) methods to tune the resolution scale and facilitate the practical aspects of a many-body calculation, and
• Efficient techniques to solve the many-body Schrödinger

An important feature of this approach is that we control the theoretical uncertainties of each aspect of a calculation, allowing us to make systematic improvements in order to reduce the theoretical error bars. While open issues remain, this provides us with a natural road map towards a predictive model of nuclei.

My own work focuses on the aforementioned RG and many-body techniques. By developing efficient new methods, my colleagues and I have extended the range of accessible nuclei from light isotopes like carbon (atomic number 6) to tin (atomic number 50) and beyond. After achieving this for isolated “magic” nuclei, the counterpart to chemistry’s noble gases, my group is now developing tools to calculate the properties of  entire chains of so-called open-shell nuclei, including those with complex intrinsic structures, and increase the number of accessible isotopes more than tenfold.

Numerical simulations are a cornerstone of my group’s research. Our calculations are performed on systems ranging from mid-size computing clusters to massively parallel supercomputers. Ensuring an intelligent use of high-performance resources is crucial in light of ever-evolving computational platforms and an explosive growth in demand. Among the challenges that we work to overcome are the massive memory requirements of three-nucleon interactions, which cannot be met by even the largest supercomputers, and the need to carry out large ensemble calculations for our uncertainty quantification efforts, which we seek to address with machine learning techniques.

My research program offers many opportunities for collaboration within the effective field theory, nuclear structure and reactions, nuclear astrophysics, and fundamental symmetries research communities, involving researchers at NSCL/FRIB and from a global network of collaborators. In pursuit of computational advances in nuclear many-body theory, we also collaborate with computer scientists, both at national facilities as well as MSU’s Department of Computational Mathematics, Science, and Engineering, providing opportunities to obtain a dual degree for interested PhD students. Moreover, my research group is involved in efforts like the NUCLEI project, a national collaborative effort between nuclear theorists, computer scientists, and mathematicians that is supported by the Department of Energy’s SciDAC (Scientific Discovery Through Advanced Computing) program.