Level structures of ^56,58Ca cast doubt on a doubly magic ⁶⁰Ca

Gamma decays were observed in ⁵⁶Ca and ⁵⁸Ca following quasi-free one-proton knockout reactions from ^57,59Sc beams at ≈200 MeV/nucleon. For ⁵⁶Ca, a γ ray transition was measured to be 1456(12) keV, while for ⁵⁸Ca an indication for a transition was observed at 1115(34) keV. Both transitions were tentatively assigned as the 2₁⁺→0_gs⁺ decays, and were compared to results from ab initio and conventional shell-model approaches. A shell-model calculation in a wide model space with a marginally modified effective nucleon-nucleon interaction depicts excellent agreement with experiment for 2₁⁺ level energies, two-neutron separation energies, and reaction cross sections, corroborating the formation of a new nuclear shell above the N = 34 shell. Its constituents, the 0f_5/2 and 0g_9/2 orbitals, are almost degenerate. This degeneracy precludes the possibility for a doubly magic ⁶⁰Ca and potentially drives the dripline of Ca isotopes to ⁷⁰Ca or even beyond.