Impurity effect of Lambda hyperon on collective excitations of atomic nuclei

TL;DR

This study explores how a Lambda hyperon influences the collective excitations of atomic nuclei, specifically in magnesium isotopes, using advanced energy density functional theory and collective Hamiltonian models.

Contribution

It provides a detailed theoretical analysis of the impurity effect of a Lambda hyperon on nuclear collective excitations, incorporating symmetry restoration and shape fluctuations.

Findings

Lambda hyperon stretches the ground state band.

Reduces B(E2) transition probability by about 9%.

Softens the potential energy surface towards spherical shape.

Abstract

Taking the ground state rotational band in $^{24}$Mg as an example, we investigate the impurity effect of $\Lambda$ hyperon on collective excitations of atomic nuclei in the framework of non-relativistic energy density functional theory. To this end, we take into account correlations related to the restoration of broken symmetries and fluctuations of collective variables by solving the eigenvalue problem of a five-dimensional collective Hamiltonian for quadrupole vibrational and rotational degrees of freedom. The parameters of the collective Hamiltonian are determined with constrained mean-field calculations for triaxial shapes using the SGII Skyrme force. We compare the low-spin spectrum for $^{24}$Mg with the spectrum for the same nucleus inside $^{25}_{\Lambda}$Mg. It is found that the $\Lambda$ hyperon stretches the ground state band and reduces the $B(E2:2^+_1 \rightarrow 0^+_1)$ value by $\sim 9%$, mainly by softening the potential energy surface towards the spherical shape, even though the shrinkage effect on the average proton radius is only $\sim0.5%$.