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

Abstract

We present a high-resolution in-beam $\gamma$-ray spectroscopy study of excited states in the mirror nuclei ${}^{55}$Co and ${}^{55}$Ni following one-nucleon knockout from a projectile beam of ${}^{56}$Ni. The newly determined partial cross sections and the $\gamma$-decay properties of excited states provide a test of state-of-the-art nuclear structure models and probe mirror symmetry in unique ways. 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 $E1$ 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 $E1$ decay pattern, however, points at stronger mixing between the two lowest $3/2^{-}$ states in ${}^{55}$Co than in its mirror ${}^{55}$Ni.