First observation of unbound ${}^{11}$O, the mirror of the halo nucleus ${}^{11}$Li

Abstract

The structure of the extremely proton-rich nucleus ${}_8^{11}$O${}_3$, the mirror of the two-neutron halo nucleus ${}_3^{11}$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: $J^{\ddot{I}}€$=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.

6 pages, three figures