The interplay of single-particle and collective motions in the low-lying states of ${}_{\mathrm{\Lambda }}^{21}$Ne 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 ${}_{\mathrm{\Lambda }}^{21}$Ne, where the configurations with the $\mathrm{\Lambda }$ hyperon occupying the first (${\mathrm{\Lambda }}_s$) and second (${\mathrm{\Lambda }}_p$) lowest-energy states are considered. The results indicate that the positive-parity states are dominated by the $\alpha {+}^{12}$C+$\alpha +{\mathrm{\Lambda }}_s$ structure. In contrast, the low-lying negative-parity states are dominated by a strong admixture of $\alpha {+}^{16}$O+${\mathrm{\Lambda }}_s$ structure and $\alpha {+}^{12}$C+$\alpha +{\mathrm{\Lambda }}_p$ 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.