Direct proton transfer on ${}^{46}$Ar supports the presence of a charge density bubble linked to a novel nuclear structure below ${}^{48}$Ca

The ${}^{46}$Ar(${}^3$He,d)${}^{47}$K reaction was performed in inverse kinematics using a radioactive ${}^{46}$Ar beam produced by the SPIRAL1 facility at GANIL and a cryogenic ${}^3$He target. The AGATA-MUGAST-VAMOS setup allowed the coincident measurement of the $\gamma$ rays, deuterons and recoiling ${}^{47}$K isotopes produced by the reaction. The relative cross sections towards the proton-addition states in ${}^{47}$K point towards a depletion of the $\pi s_{1\mathrm{/}2}$ shell. The experimental findings are in good agreement with ab initio calculations, which predict that ${}^{46}$Ar exhibits a charge density bubble associated with a pronounced proton closed-shell character.