Pairing Forces Govern Population of Doubly Magic Ca54 from Direct Reactions

Direct proton-knockout reactions of $^{55}\mathrm{Sc}$ at $\ensuremath{\sim}220\text{ }\text{ }\mathrm{MeV}/\mathrm{nucleon}$ were studied at the RIKEN Radioactive Isotope Beam Factory. Populated states of $^{54}\mathrm{Ca}$ were investigated through $\ensuremath{\gamma}$-ray and invariant-mass spectroscopy. Level energies were calculated from the nuclear shell model employing a phenomenological internucleon interaction. Theoretical cross sections to states were calculated from distorted-wave impulse approximation estimates multiplied by the shell model spectroscopic factors, which describe the wave function overlap of the $^{55}\mathrm{Sc}$ ground state with states in $^{54}\mathrm{Ca}$. Despite the calculations showing a significant amplitude of excited neutron configurations in the ground-state of $^{55}\mathrm{Sc}$, valence proton removals populated predominantly the ground state of $^{54}\mathrm{Ca}$. This counterintuitive result is attributed to pairing effects leading to a dominance of the ground-state spectroscopic factor. Owing to the ubiquity of the pairing interaction, this argument should be generally applicable to direct knockout reactions from odd-even to even-even nuclei.