Beyond the Neutron Drip-Line: The Unbound Oxygen Isotopes 25O and 26O
  The very neutron-rich oxygen isotopes 25O and 26O are investigated experimentally and theoret- ically. In this first R3B-LAND experiment, the unbound states are populated at GSI via proton- knockout reactions from 26F and 27F at relativistic energies around 450 MeV/nucleon. From the kinematically complete measurement of the decay into 24O plus one or two neutrons, the 25O ground- state energy and lifetime are determined, and upper limits for the 26O ground state are extracted. In addition, the results provide evidence for an excited state in 26O at around 4 MeV. The ex- perimental 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. 
C. Caesar (1 and 2), J. Simonis (1 and 3), T. Adachi (4), Y. Aksyutina   (2 and 3), J. Alcantara (5), S. Altstadt (6), H. Al...
nucl-ex nucl-th