Examining the N = 28 shell closure through high-precision mass measurements of Ar 46 – 48

The strength of the $N=28$ magic number in neutron-rich argon isotopes is examined through high-precision mass measurements of $^{46\text{--}48}\mathrm{Ar}$, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The new mass values are up to 90 times more precise than previous measurements. While they suggest the persistence of the $N=28$ shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth of spectroscopic data and theoretical investigations performed with the SDPF-U phenomenological shell model interaction. Our results are also compared with ab initio calculations using the valence space in-medium similarity renormalization group and the self-consistent Green's function approaches. Both calculations provide a very good account of mass systematics at and around $Z=18$ and, generally, a consistent description of the physics in this region. This combined analysis indicates that $^{46}\mathrm{Ar}$ is the transition between the closed-shell $^{48}\mathrm{Ca}$ and collective $^{44}\mathrm{S}$.