Spectroscopic Study Of Lambda Hyperon Impurity Effect In Atomic Nuclei Based On Covariant Density Functional Theory

The impurity effect of hyperons in atomic nuclei has received a renewed interest in nuclear physics since the first experimental observation of appreciable reduction of E2 transition strength in low-lying states of the hypernucleus 7ΔLi. Many more data on low-lying states of A hypernuclei will be measured soon for sd-shell nuclei, providing good opportunities to study the A impurity effect. The spectroscopy of hypernuclei has become one of the most important subjects in nuclear physics. In recent years, covariant density functional theory (CDFT) has achieved a great success in describing nuclear structural properties and it has also been extended for the studies of hypernuclei.In this thesis, we extend a triaxially deformed CDFT based on point-coupling energy density functional by including Lambda hyperon degree of freedom, develop a new code for triaxially deformed hypernuclei and carry out a quantitative analysis of the A hyperon impurity effect on the low-lying states of sd-shell nuclei, where the Λ hyperon is injected into the lowest positive-(Λs) and negative-(Λp) parity states.Specifically, we adopt our newly-developed code to calculate Λ binding energies and potential-energy surfaces (PESs) of hypernuclei in deformations (β,γ) plane. The PESs for the Λ hypernuclei with good angular momentum are also obtained by using a microscopic particle rotor model with the same relativistic EDF. By solving a five-dimensional collective Hamiltonian for the collective excitations of triaxially deformed core nuclei, we analyze the impurity effects of Λs and Λp on the low-lying states of the core nuclei 24,26Mg and 30Si. We find that the Λs increases the excitation energy of the 21+ state and decreases the E2 transition strength from this state to the ground state by 12% to 17%, significantly larger than the predictions based on nonrelativistic approaches. However, the Λp tends to develop hypernuclear deformation. Our studies indicate that the dynamical correlations due to symmetry restoration and shape fluctuation are important in order to study the Λ impurity effect in a quantitative way.