Heiko Hergert

In August 2017, it was announced that Dr. Hergert was selected as one of 59 researchers nationwide to receive a 2017 U.S. Department of Energy Early Career Research Program Award.

Heiko Hergert joined the NSCL in 2014 and became an Assistant Professor in the Department of Physics and Astronomy in Fall 2015. He describes his research interests and projects as follows.

Atomic nuclei are among nature's most fascinating, and at the same time, most confounding objects. This is mainly due to the complicated nature of the strong nuclear interaction. The fundamental theory of the strong interaction is Quantum Chromodynamics (QCD), which describes interactions of quarks and gluons. While deceptively easy to write down, QCD is hard to solve. Describing nuclei based on QCD is even more challenging because nucleons are composite objects that have a complicated quark and gluon substructure themselves.

The interplay of the complicated nuclear interactions with quantum-mechanical many-body effects gives rise to a rich variety of nuclear phenomena. This is especially true for exotic nuclei in the more remote regions of the nuclear chart. These nuclei are the focus of the experimental program at FRIB, and a reliable theoretical framework is required to analyze and guide future experiments.

Ab initio (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 QCD,
* renormalization group (RG) methods to systematically dial the resolution scale and facilitate the practical aspects of a many-body calculation, and
* efficient techniques to solve the many-body Schrödinger equation.

An important feature of this approach is that we control the theoretical uncertainties of each ingredient; i.e., we can provide theoretical error bars. The theoretical uncertainties can be reduced by systematically improving the various aspects of a calculation. While open issues remain, this approach provides a road map towards a truly predictive model of nuclei.

Personally, I work primarily on the RG and many-body aspects of the ab initio framework. I am one of the main developers of efficient new many-body methods that have pushed the range of accessible nuclei from light isotopes like carbon (with atomic number 6) to tin (atomic number 50). This was first achieved for isolated “magic” nuclei, nuclear physics' counterpart to the noble gases of chemisty. I am now developing tools to calculate the properties of entire chains of so-called open-shell nuclei. This will increase the number of accessible isotopes more than tenfold.

The growing reach of ab initio many-body methods opens up many opportunities for collaboration with my experimental colleagues at NSCL/FRIB, and projects for prospective Ph.D. students can be tied to the theoretical analysis and the planning of experiments. In turn, the confrontation of our calculations with new data will allow us to diagnose potential problems with the chiral interactions we use, and provide guidance to their constructors.

Computation is an important aspect of my work. Calculations are necessarily parallelized, and run on mid-size computing clusters or massively parallel systems, depending on the specific problem. Thus, one challenges we face is to ensure good use of the continuously changing architectures of high-performance computers. However, it is not enough to focus on improvement through hardware adaptation alone. For instance, one of the biggest obstacles we face in ab initio calculations are the massive memory requirements of three-nucleon interactions, which cannot be met by even the largest supercomputers. Consequently, I am also interested in improving the algorithms and numerical techniques we use to overcome such limitations and maximize numerical efficiency.