Abstract
The in-medium similarity renormalization group (IMSRG) is a powerful and flexible many-body method to compute the structure of nuclei starting from nuclear forces. Recent developments have extended the IMSRG from its standard truncation at the normal-ordered two-body level, the IMSRG(2), to a precision approximation including normal-ordered three-body operators, the IMSRG(3)-N7. This improvement provides a more precise solution to the many-body problem and makes it possible to quantify many-body uncertainties in IMSRG calculations. We explore the structure of 44,48,52Ca using the IMSRG(3)-N7, focusing on understanding existing discrepancies of the IMSRG(2) to experimental results. We find a significantly better description of the first 2+ excitation energy of 48Ca, improving the description of the shell closure at N = 28. At the same time, we find that the IMSRG(3)-N7 corrections to charge radii do not resolve the systematic underprediction of the puzzling large charge radius difference between 52Ca and 48Ca. We present estimates of many-body uncertainties of IMSRG(2) calculations applicable also to other systems based on the size extensivity of the method.
