Why oxygen-28 is unbound despite magic proton and neutron numbers
In plain words
Oxygen-28 has 8 protons and 20 neutrons, both 'magic' numbers that usually make nuclei extra stable, yet it has been known since the late 1990s not to hold together; a 2023 experiment observed it directly as a short-lived state. Explaining this tests how shell structure changes far from stable nuclei.
Precise statement
28O was observed in 2023 as a state unbound with respect to $24\mathrm{O} + 4\mathrm{n}$ (Kondo et al., Nature 620, 965, 2023), without evidence of an $N = 20$ shell closure; fragmentation searches had found no bound 28O since 1997. Determine the mechanism (continuum coupling, 3N forces, deformation, intruder pf-shell configurations) that removes $N = 20$ magicity and places the oxygen drip line at 24O. Answer: an ab initio calculation with quantified uncertainty reproducing the 28O decay energy and 24O as the last bound isotope, with the dominant contribution identified.
What would settle it
Ab initio continuum calculations reproducing the measured 28O and 26O energies together with the 24O drip line using one interaction.
Status in the literature
Three-nucleon forces explain the drip line at 24O (Otsuka et al., 2010); ab initio calculations did not agree on the structure of 28O relative to the 2023 data.