How Robust is the N = 34 Subshell Closure? First Spectroscopy of ${}^{52}$Ar

The first $\gamma$-ray spectroscopy of ${}^{52}$Ar, with the neutron number N = 34, was measured using the ${}^{53}$K(p,2p) one-proton removal reaction at $\sim$210 MeV/u at the RIBF facility. The 2${}_1^{+}$ excitation energy is found at 1656(18) keV, the highest among the Ar isotopes with N $>$ 20. This result is the first experimental signature of the persistence of the N = 34 subshell closure beyond ${}^{54}$Ca, i.e., below the magic proton number Z = 20. Shell-model calculations with phenomenological and chiral-effective-field-theory interactions both reproduce the measured 2${}_1^{+}$ systematics of neutron-rich Ar isotopes, and support a N = 34 subshell closure in ${}^{52}$Ar.