A recent ${}^{47}\mathrm{K}(d,p\gamma ){}^{48}\mathrm{K}$ transfer reaction measurement identified new excited states in ${}^{48}\mathrm{K}$ and extracted the corresponding spectroscopic factors (SFs) [C. J. Paxman et al., Phys. Rev. Lett. 134, 162504 (2025)], but exposed sizable discrepancies with large-scale shell-model (LSSM) calculations—especially for the low-lying states—suggesting shortcomings in the proton-neutron interaction employed by the LSSM. In this work, we revisit the low-lying states and SFs of ${}^{48}\mathrm{K}$ using the ab initio valence-space in-medium similarity renormalization group (VS-IMSRG) approach based on the chiral two- and three-nucleon forces. The calculated excitation energies reproduce the experimental data for ${}^{48}\mathrm{K}$, whereas computed SFs systematically exceed experimental values. We trace this overestimation to missing reduction factors that account for nonidealities of the transfer reaction. After introducing a phenomenological reduction factor, our VS-IMSRG results and the LSSM calculations achieve agreement with experiment. We also perform the same analysis for the neutron SFs of ${}^{47}\mathrm{Ar}$. Furthermore, we extend the ab initio calculations across the $N=28$ isotones, computing excitation energies and single-neutron transfer SFs from $N=29$ isotones ranging from ${}^{48}\mathrm{K}$ to ${}^{45}\mathrm{S}$. By systematically removing protons from ${}^{48}\mathrm{K}$ to ${}^{45}\mathrm{S}$, we trace the evolution of the $N=28$ shell strength via theoretical SFs values. Our results provide a microscopic pathway to quantify the weakening of the $N=28$ shell closure.

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• Nuclear many-body theory
• Nuclear reactions
• Spectroscopic factors & electromagnetic moments
• Transfer reactions
• 39 ≤ A ≤ 58
• Ab initio calculations