The unbound nucleus ${}^{26}\mathrm{O}$ has been investigated using invariant-mass spectroscopy following one-proton removal reaction from a ${}^{27}\mathrm{F}$ beam at $201\mathrm{MeV}/\mathrm{nucleon}$. The decay products, ${}^{24}\mathrm{O}$ and two neutrons, were detected in coincidence using the newly commissioned SAMURAI spectrometer at the RIKEN Radioactive Isotope Beam Factory. The ${}^{26}\mathrm{O}$ ground-state resonance was found to lie only $18\pm 3(\mathrm{stat})\pm 4(\mathrm{syst})\mathrm{keV}$ above threshold. In addition, a higher lying level, which is most likely the first $2^{+}$ state, was observed for the first time at ${1.28}_{-0.08}^{+0.11}\mathrm{MeV}$ above threshold. Comparison with theoretical predictions suggests that three-nucleon forces, $pf$-shell intruder configurations, and the continuum are key elements to understanding the structure of the most neutron-rich oxygen isotopes beyond the drip line.

• Received 27 August 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.102503