Mass Measurement of Upper fp -Shell N=Z−2 and N=Z

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 ($\ensuremath{\delta}{V}_{pn}$) 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 $\ensuremath{\delta}{V}_{pn}$ 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 $\ensuremath{\delta}{V}_{pn}$ in even-even and odd-odd nuclei.