Quadrupole collectivity beyond N = 28: intermediate-energy Coulomb excitation of (47,48)Ar

We report on the first experimental study of quadrupole collectivity in the very neutron-rich nuclei (47,48)Ar using intermediate-energy Coulomb excitation. These nuclei are located along the path from doubly magic Ca to collective S and Si isotopes, a critical region of shell evolution and structural change. The deduced B(E2) transition strengths are confronted with large-scale shell-model calculations in the sdpf shell using the state-of-the-art SDPF-Uand EPQQM effective interactions. The comparison between experiment and theory indicates that a shell-model description of Ar isotopes around N=28 remains a challenge.