Structure of three-body hypernuclei

Abstract

Hypernuclear physics offers a unique playground for understanding the physics of the strong interaction beyond the up- and down-quark sector. A particularly attractive feature is that hyperons, particles containing at least one strange quark, offer us an opportunity to probe nuclear interior without being affected by the Pauli principle. Due to the lack of two-body bound systems, three-body NNΛ systems take a prominent role for understanding hypernuclear physics. In recent years the focus has been on two particular systems, the hypertriton and the Λnn system in the isospin I=0 and I=1 sector respectively. Although the first has been known for many decades it is still not fully understood. Recent results by the STAR Collaboration question the binding energy established in the past. In addition experimental results for the hypertriton width vary over a large range and are therefore inconsistent. The hypertriton is considered to be a shallow S-wave bound state with a Λ separation energy of a few hundred keV at most. The nature of the Λnn, however, remains controversial. In 2013 first results have shown evidence that this system might be bound, however the possibility of a resonance or a virtual state are also up for debate. Since typical momenta of these systems are small compared to the pion mass, we can utilize pionless effective field theory to explore these systems. In the first part of this thesis we exploit this separation of scales within the systems in an effective field theory approach analyzing the structure of these hypernuclei. Effective field theory offers a unique model-independent approach with controllable uncertainties. Utilizing this method we calculate scattering properties for both systems. Since the scattering lengths of the two-body systems are large, both systems show universal behavior due to the Efimov effect. We therefore calculate universal relations for both systems, connecting different observables. In a next step we extend our efforts by calculating matter radii and wave functions. In the second part of this work, we focus on the other open question about the lifetime of the hypertriton against the weak interaction. We calculate the four most important mesonic decay channels in a fundamental deuteron approximation. For this part, we utilize isospin symmetry to connect charged and uncharged channels. In light of the new resuls for the binding energy, we discuss the lifetime and related properties relating binding energy data to lifetime date. In addition we take recent results for the weak Λ decay parameters into consideration.