Background: The impurity effect of hyperons on 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 Λ7Li. Many more data on low-lying states of Λ hypernuclei will be measured soon for sd-shell nuclei, providing good opportunities to study the Λ impurity effect on nuclear low-energy excitations. Purpose: We carry out a quantitative analysis of the Λ hyperon impurity effect on the low-lying states of sd-shell nuclei at the beyond-mean-field level based on a relativistic point-coupling energy density functional (EDF), considering that the Λ hyperon is injected into the lowest positive-parity (Λs) and negative-parity (Λp) states. Method: We adopt a triaxially deformed relativistic mean-field (RMF) approach for hypernuclei and calculate the Λ binding energies of hypernuclei as well as the potential-energy surfaces (PESs) in the (β,γ) deformation plane. We also calculate the PESs for the Λ hypernuclei with good quantum numbers by using a microscopic particle rotor model (PRM) with the same relativistic EDF. The triaxially deformed RMF approach is further applied in order to determine the parameters of a five-dimensional collective Hamiltonian (5DCH) for the collective excitations of triaxially deformed core nuclei. Taking MgΛ25,27 and SiΛ31 as examples, we analyze the impurity effects of Λs and Λp on the low-lying states of the core nuclei. Results: We show that Λs increases the excitation energy of the 21+ state and decreases the E2 transition strength from this state to the ground state by 12%to17%. On the other hand, Λp tends to develop pronounced energy minima with larger deformation, although it modifies the collective parameters in such a way that the collectivity of the core nucleus can be either increased or decreased. Conclusions: The quadrupole deformation significantly affects the Λ binding energies of deformed hypernuclei. A beyond-mean-field approach with the dynamical correlations due to restoration of broken symmetries and shape fluctuation is essential in order to study the Λ impurity effect in a quantitative way.
ASJC Scopus subject areas
- Nuclear and High Energy Physics