Session Y05: Weakly-Bound Systems

Precision measurements in light, weakly bound nuclei

Precision measurements of the properties of light ($A \leq 12$) radioactive nuclei offer the opportunity to study nuclear forces under extreme neutron-to-proton ratios in relatively simple systems that are accessible to high-precision {\em ab-initio} nuclear theory. These measurements guide the development of effective models to accurately describe nuclear structure and reactions. Due to their weakly bound character, these nuclei also exhibit rich phenomenology, of which the so-called halo nuclei are perhaps the most remarkable: nuclei that consist of a core plus a halo of dilute nuclear matter with unusually large extend. For example, $^{11}$Li and $^{11}$Be are archetypes of two-neutron and one-neutron halo nuclei, respectively. I will provide a (selective) overview of recent measurements of these light nuclei that push the precision frontier and use a combination of electromagnetic, strong, and weak probes. Examples are laser spectroscopic measurements that use tiny energy shifts of bound electronic states to extract nuclear ground state properties, nuclear reaction studies using gamma spectroscopy to probe the wave functions of excited states, and beta decay measurements that exploit the relative simplicity of these isotopes to push the limit of knowledge of the weak interaction and search for new physics.   

Searches for beyond-the-Standard Model physics with light nuclei

Low-energy precision experiments can indirectly probe high energy scales and are complementary to collider searches for beyond-the-Standard Model (BSM) physics. Low-energy experiments, however, often involve complicated nuclei which muddles the interpretation of the data. On the other hand, light nuclei are simple enough to be described with firm theoretical tools such as (chiral) effective field theory. Precision experiments involving light nuclei are therefore both theoretically and experimentally interesting. I discuss various (proposed) precision tests involving light nuclei focusing on electric dipole moments and dark matter direct detection. I focus on what these experiments can teach us about BSM physics and about experiments involving more complicated nuclei.   

Effective field theory for halo nuclei

The region towards the proton and neutron drip lines in the isotopic chart, where nuclei become less bound, has attracted strong interest from the nuclear physics community. In some of those nuclei, known as halo nuclei, a clusterized picture emerges: the distances between one or a few nucleons and core, or among clusters, are much larger than the range of strong interaction. In the past two decades, the effective field theory (EFT) methodology, which treats clusters as fundamental degrees of freedom (DOF) and capitalizes on the separation of scales, has been intensively studied and applied to describe halo nuclei. This EFT is closely related to the EFT that describes “hadronic molecules" and cold atomic gases. The methodology is systematical and model-independent. In this talk, I will discuss some of the recent developments in this field, and focus on the progress our group has made in studying nuclear reactions using EFT, including elastic and inelastic scatterings and radiative capture reactions. I will highlight how the marriage between EFT and the statistical analysis tool, Bayesian inference, produces robust uncertainty estimations for extrapolated reaction cross sections and previously unrealized constraints on scattering observables. In the end, I will discuss the complementarity between EFT and numerically intensive ab initio structure calculations that use nucleons as fundamental DOF, and how EFT can help ab initio methods calculate scattering and reactions.