Abstract
The very neutron-rich oxygen isotopes O-25 and O-26 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 F-26 and F-27 at relativistic energies around 442 and 414
MeV/nucleon, respectively. From the kinematically complete measurement
of the decay into O-24 plus one or two neutrons, the O-25 ground-state
energy and width are determined, and upper limits for the O-26
ground-state energy and lifetime are extracted. In addition, the results
provide indications for an excited state in O-26 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.
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