Electromagnetic properties of light nuclei with chiral effective field theory currents

Abstract

The ability to describe electromagnetic properties of nuclei is fundamental to our understand- ing of nuclear structure and dynamics. Experimental methods that measure these properties enable a clean way to isolate the nuclear physics content, because the relatively weak and well understood electromagnetic interaction is perturbative in nature and thus appropriately described. In this thesis we study electromagnetic properties of light nuclei within the framework of chiral effective field theory (EFT). The modern approach to low-energy nuclear physics is formulated by chiral EFT which describes the nucleus in terms of nucleon and pion degrees of freedom based on the symmetries of the underlying fundamental theory of quantum chromo- dynamics. It provides a systematically improvable calculation scheme and permits a unified description of the strong-interaction dynamics between nucleons and the interaction with an external probe. The nuclear component of such an interaction is described by nuclear currents. Both nuclear interactions and currents are consistently derived within chiral EFT and exhibit a naturally emerging many-body operator structure. Recent progress on the development of nuclear interactions and nuclear currents have set the stage for high-precision calculations complemented with systematic truncation uncertainty estimates. We study the deuteron, the triton, and the helion electromagnetic form factors with two- and three-nucleon chiral interactions developed in an order-by-order manner which allows us to compute the associated truncation uncertainty estimates. We find good agreement at low momentum transfers for the charge form factors and a consistent description of the experimental first minimum once the uncertainty estimates are incorporated. For the tri- nucleon magnetic form factors we find that leading two-body currents (2BCs), which arise from the exchange of a pion between a pair of nucleons, lead to better agreement with data over the entire momentum-transfer region. To obtain insights into the effect of various chiral interactions with and without three-nucleon forces and to quantify the impact of 2BCs on the zero-momentum-transfer region, we analyze the magnetic moments and the electromagnetic radii of these light nuclei. We observe that three-nucleon forces reduce the radii slightly and have a negligible effect on the magnetic moment, while 2BCs significantly modify both the magnetic radius and magnetic moment indicating that the exchange dynamics between the nucleons are essential for magnetic observables. As a first step towards a consistent study of other light nuclei, we examine the magnetic moment and a magnetic transition of 6Li which is the next light nucleus after the three- nucleon nuclei with nonvanishing magnetic ground-state properties. To achieve this, we include contributions to the magnetic dipole operator beyond leading order which arise from the leading 2BCs and we employ similarity renormalization group evolved chiral interactions to enhance the many-body convergence. Our results are in remarkable agreement with a new precision experiment after consistently evolving and including 2BCs to the magnetic dipole operator, thus advancing our understanding of nuclear interactions and electromagnetic currents in many-nucleon systems.