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

## TL;DR

This study investigates the nuclear structure and mirror symmetry in $^{55}$Co and $^{55}$Ni through knockout reactions from $^{56}$Ni, revealing asymmetries in cross sections and decay patterns that challenge current models.

## Contribution

It provides new experimental data on excited states and decay properties in mirror nuclei, highlighting asymmetries that test and challenge existing nuclear structure theories.

## Key findings

- Identified mirror asymmetry in partial cross sections to low-lying states.
- Observed significant differences in $E1$ decay patterns between mirror nuclei.
- Revealed that current shell-model descriptions cannot fully explain the asymmetries.

## 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.