Ab initio calculations of anomalous seniority breaking in the πg9/2 shell for the N = 50 isotones

We performed ab initio valence-space in-medium similarity renormalization group (VS-IMSRG) calculations based on chiral two-nucleon and three-nucleon interactions to investigate the anomalous seniority breaking in the neutron number $N=50$ isotones: ⁹²Mo, ⁹⁴Ru, ⁹⁶Pd, and ⁹⁸Cd. Our calculations well reproduced the measured low-lying spectra and electromagnetic $E2$ transitions in these nuclei, supporting partial seniority conservation in the first π g9/2 shell. Recent experiments have revealed that, compared to the symmetric patterns predicted under the conserved seniority symmetry, the 4⁺₁→2⁺₁ $E2$ transition strength in ⁹⁴Ru is significantly enhanced and that in ⁹⁶Pd is suppressed. In contrast, the 6⁺₁→ 4⁺₁ and 8⁺₁→6⁺₁ transitions exhibit the opposite trend. We found that this anomalous asymmetry is sensitive to subtle seniority breaking effects, providing a stringent test for state-of-the-art nucleon-nucleon interactions and nuclear models. We analyzed the anomalous asymmetry using VS-IMSRG calculations across various valence spaces. Our ab initio results suggest that core excitations of both proton and neutron across the $Z=50$ shell are ascribed to the observed anomalous seniority breaking in the $N=50$ isotones.