Brandon Lem

The quantum many-body problem (QMBP) plays a central role in theoretical low-energy nuclear physics, where the aim is to predict the properties of atomic nuclei (such as radii and binding energies) and the nuclear matter found inside stars, starting from the constituent nucleons (protons and neutrons). The QMBP entails solving the Schrodinger equation for a large number of interacting particles. In low-energy nuclear physics, this already challenging task is further complicated by the fact that the interactions between nucleons are strong, poorly known at short distances, and exhibit sizeable three- and higher-body forces.

A number of approximate but systematically improvable methods for solving the QMBP have been developed in physics and chemistry since the 1960s. I am exploring how one recent method---the in-medium similarity renormalization group (IMSRG)---can provide microscopic calculations of the nuclear matter equation-of-state (EOS), which has important implications for neutron star physics and refining our understanding of nuclear forces. This will entail non-trivial extensions to carry out EOS calculations at finite temperatures, and to carry out calculations of nuclear matter response functions (e.g., dynamic structure factors). To accomplish this goal, I am developing renormalization group-inspired dimensionality reduction techniques to carry out large-scale IMSRG EOS calculations with quantified uncertainties.

An improved understanding of the EOS will refine our parameterizations of nuclear force models. For instance, nuclear forces derived from chiral effective field theory are known to give predictions that rapidly degrade beyond light nuclei, with significant observed over-binding and charge-radii that are much smaller than experimental values. These failures are tied to the poor description of empirical nuclear matter saturation properties due to most chiral interactions being fit to light nuclei. It is therefore likely that incorporating nuclear matter properties into the optimization procedure will improve the performance of chiral interactions for heavier nuclei.