  The strength of the $N$ = 28 magic number in neutron-rich argon isotopes is
examined through high-precision mass measurements of $^{46-48}$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 \emph{SDPF-U} phenomenological
shell model interaction. Our results are also compared with \emph{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 $^{46}$Ar is the transition between the closed-shell $^{48}$Ca
and collective $^{44}$S.

Nuclear Experiment;Nuclear Theory