Examining the N=28 shell closure through high-precision mass measurements of Ar46–48

The strength of theN=28 magic number in neutron-rich argon isotopes is examined through high-precisionmass measurements of46–48Ar, performed with the ISOLTRAP mass spectrometer at ISOLDE/CERN. The newmass values are up to 90 times more precise than previous measurements. While they suggest the persistenceof theN=28 shell closure for argon, we show that this conclusion has to be nuanced in light of the wealth ofspectroscopic data and theoretical investigations performed with the SDPF-U phenomenological shell modelinteraction. Our results are also compared withab initiocalculations using the valence space in-mediumsimilarity renormalization group and the self-consistent Green’s function approaches. Both calculations providea very good account of mass systematics at and aroundZ=18 and, generally, a consistent description of thephysics in this region. This combined analysis indicates that46Ar is the transition between the closed-shell48Caand collective44S.