One-proton and one-neutron knockout reactions from N = Z = 28 56 Ni to the A = 55 mirror pair 55 Co and 55 Ni

We present a high-resolution in-beam γ -ray spectroscopy study of excited states in the mirror nuclei 55 Co and 56 Ni following one-nucleon knockout from a projectile beam of 56 Ni. The newly determined partial cross sections and the γ -decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. The new experimental data are compared to large-scale shell-model calculations in the full pf space and including charge-dependent contributions. A mirror asymmetry for the partial cross sections leading to the two lowest 3/2 − states in the A = 55 mirror pair was identified as well as a significant difference in the E 1 decays from the 1/2 1 + state to the same two 3/2 − states. The mirror asymmetry in the partial cross sections cannot be reconciled with the present shell-model picture or small mixing introduced in a two-state model. The observed mirror asymmetry in the E decay pattern, however, points at stronger mixing between the two lowest 3/2 − states in 55 Co than in its mirror 55 Ni.