The structure of the extremely proton-rich nucleus $^{ 11}_{ ~8}$O$_3$, the mirror of the two-neutron halo nucleus $^{ 11}_{ ~3}$Li$_8$, has been studied experimentally for the first time. Following two-neutron knockout reactions with a $^{ 13}$O beam, the $^{ 11}$O decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width $\sim$3\,MeV was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four-lowest $^{ 11}$O resonant states: $J^{ \pi}$=3/2$^-_1$, 3/2$^-_2$, 5/2$^+_1$, and 5/2$^+_2$. The widths and configurations of these states show strong, non-monotonic dependencies on the depth of the $p$-$^9$C potential. This unusual behavior is due to the presence of a broad threshold resonant state in $^{ 10}$N, which is an analog of the virtual state in $^{ 10}$Li in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of $^{ 11}$O and $^{ 11}$Li was found.
Categories: nucl-ex nucl-th