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

## TL;DR

This study reports the first experimental observation of the unbound proton-rich nucleus $^{11}$O, revealing its resonant states and their complex behavior, and compares it to its neutron-rich mirror $^{11}$Li.

## Contribution

First experimental investigation of $^{11}$O, providing insights into its resonant states and isospin symmetry with $^{11}$Li using a Gamow coupled-channel approach.

## Key findings

- Observed a broad peak of ~3 MeV in $^{11}$O decay spectrum.
- Identified contributions from four low-lying resonant states with specific spins and parities.
- Found strong, non-monotonic dependence of state properties on potential depth due to a broad threshold resonance.

## Abstract

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.