N=8 Shell Breaking in ^{12}Be from a Single-Particle Perspective

Experimental observations of the low-lying states in ^{12}Be and their accurate modeling play an essential role in understanding the disappearance of the N=8 magic number. Long-standing experimental ambiguities have been clarified using an one-neutron adding (d, p) reaction on ^{11}Be using the ISOLDE Solenoidal Spectrometer at CERN's HIE-ISOLDE facility. The single-particle energies of 1s_{1/2}, 0d_{5/2}, and 0p_{1/2} orbitals in ^{12}Be have been determined from the extracted spectroscopic factors. A significant reduction between the separation of 1s_{1/2} and 0p_{1/2} orbitals is found in comparison with the carbon isotones, highlighting the breakdown of the N=8 shell. These observations serve as an important test of different effects incorporated in theoretical models. It is found that two synergistic mechanisms, core deformation and weak binding, are responsible for the N=8 shell breaking and the exotic near-threshold phenomena observed in ^{12}Be, including the narrow unnatural-parity resonance 0_{1}^{-} and the possible halolike nature of the 0_{2}^{+} isomer.