Beyond the neutron drip line: The unbound oxygen isotopes<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mn>25</mml:mn></mml:msup></mml:math>O and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mn>26</mml:mn></mml:msup></mml:math>O

The very neutron-rich oxygen isotopes ${}^{25}$O and ${}^{26}$O are investigated experimentally and theoretically. The unbound states are populated in an experiment performed at the R3B-LAND setup at GSI via proton-knockout reactions from ${}^{26}$F and ${}^{27}$F at relativistic energies around 442 and 414 MeV/nucleon, respectively. From the kinematically complete measurement of the decay into ${}^{24}$O plus one or two neutrons, the ${}^{25}$O ground-state energy and width are determined, and upper limits for the ${}^{26}$O ground-state energy and lifetime are extracted. In addition, the results provide indications for an excited state in ${}^{26}$O at around 4 MeV. The experimental findings are compared to theoretical shell-model calculations based on chiral two- and three-nucleon ($3N$) forces, including for the first time residual $3N$ forces, which are shown to be amplified as valence neutrons are added.