The interplay of single-particle and collective motions in the low-lying states of _Λ ^21 with quadrupole-octupole correlations

The beyond mean-field approach for low-lying hypernuclear states is extended by mixing the configurations associated with both single-particle and quadrupole-octupole collective excitations within the generator coordinate method based on a covariant density functional theory. The method is demonstrated in the application to the low-lying states of _Λ ^21 , where the configurations with the Λ hyperon occupying the first (Λ s ) and second (Λ p ) lowest-energy states are considered. The results indicate that the positive-parity states are dominated by the α + 12 C + α + Λ s structure. In contrast, the low-lying negative-parity states are dominated by a strong admixture of α + 16 O + Λ s structure and α + 12 C + α + Λ p structure due to the inclusion of octupole correlations. As a result, the low-lying negative-parity states become much lower than what is expected from the previous studies without the mixing, and the electric multipole transition strengths are significantly quenched.