Using a novel method of isochronous mass spectrometry, the masses of ${}^{62}\mathrm{Ge}$, ${}^{64}\mathrm{As}$, ${}^{66}\mathrm{Se}$, and ${}^{70}\mathrm{Kr}$ are measured for the first time, and the masses of ${}^{58}\mathrm{Zn}$, ${}^{61}\mathrm{Ga}$, ${}^{63}\mathrm{Ge}$, ${}^{65}\mathrm{As}$, ${}^{67}\mathrm{Se}$, ${}^{71}\mathrm{Kr}$, and ${}^{75}\mathrm{Sr}$ are redetermined with improved accuracy. The new masses allow us to derive residual proton-neutron interactions ($\delta {V}_{pn}$) in the $N=Z$ nuclei, which are found to decrease (increase) with increasing mass $A$ for even-even (odd-odd) nuclei beyond $Z=28$. This bifurcation of $\delta {V}_{pn}$ cannot be reproduced by the available mass models, nor is it consistent with expectations of a pseudo-SU(4) symmetry restoration in the $fp$ shell. We performed *ab initio* calculations with a chiral three-nucleon force (3NF) included, which indicate the enhancement of the $T=1$ $pn$ pairing over the $T=0$ $pn$ pairing in this mass region, leading to the opposite evolving trends of $\delta {V}_{pn}$ in even-even and odd-odd nuclei.

- Received 24 November 2022
- Revised 1 March 2023
- Accepted 17 March 2023

DOI:https://doi.org/10.1103/PhysRevLett.130.192501

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Nuclear Physics