Shape of 44 Ar: onset of deformation in neutron-rich nuclei 48 Ca

The development of deformation and shape coexistence in the vicinity of doubly magic 48 Ca, related to the weakening of the N = 28 shell closure, was addressed in a low-energy Coulomb excitation experiment using a radioactive 44 Ar beam from the SPIRAL facility at GANIL. The 2 + 1 and 2 + 2 states in 44 Ar were excited on 208 Pb and 109 Ag targets at two different beam energies. B(E2) values between all observed states and the spectroscopic quadrupole moment of the 2 + 1 state were extracted from the differential Coulomb excitation cross sections, indicating a prolate shape of the 44 Ar nucleus and giving evidence of an onset of deformation already two protons and two neutrons away from doubly magic 48 Ca. New Hartree-Fock-Bogoliubov based configuration mixing calculations have been performed with the Gogny D1S interaction for 44 Ar and neighboring nuclei using two different approaches: the angular momentum projected generator coordinate method considering axial quadrupole deformations and a five-dimensional approach including the triaxial degree of freedom. The experimental values and new calculations are furthermore compared to shell-model calculations and to relativistic mean-field calculations. The new results give insight into the weakening of the N = 28 shell closure and the development of deformation in this neutron-rich region of the nuclear chart.