First observation of (28)O

Subjecting a physical system to extreme conditions is one of the means often used to obtain a better understanding and deeper insight into its organization and structure. In the case of the atomic nucleus, one such approach is to investigate isotopes that have very different neutron-to-proton (N/Z) ratios than in stable nuclei. Light, neutron-rich isotopes exhibit the most asymmetric N/Z ratios and those lying beyond the limits of binding, which undergo spontaneous neutron emission and exist only as very short-lived resonances (about 10(−21) s), provide the most stringent tests of modern nuclear-structure theories. Here we report on the first observation of (28)O and (27)O through their decay into (24)O and four and three neutrons, respectively. The (28)O nucleus is of particular interest as, with the Z = 8 and N = 20 magic numbers(1,2), it is expected in the standard shell-model picture of nuclear structure to be one of a relatively small number of so-called ‘doubly magic’ nuclei. Both (27)O and (28)O were found to exist as narrow, low-lying resonances and their decay energies are compared here to the results of sophisticated theoretical modelling, including a large-scale shell-model calculation and a newly developed statistical approach. In both cases, the underlying nuclear interactions were derived from effective field theories of quantum chromodynamics. Finally, it is shown that the cross-section for the production of (28)O from a (29)F beam is consistent with it not exhibiting a closed N = 20 shell structure.