Ab initio calculations of anomalous seniority breaking in the $\pi g_{9/2}$ shell for the $N=50$ isotones

TL;DR

This paper uses advanced ab initio calculations to explore unexpected seniority breaking phenomena in N=50 isotones, revealing the importance of core excitations and providing insights into nuclear structure beyond traditional models.

Contribution

It demonstrates that core excitations across the Z=50 shell are crucial for understanding anomalous seniority breaking, advancing nuclear structure modeling with ab initio methods.

Findings

Reproduced low-lying spectra and E2 transitions accurately

Identified sensitivity of asymmetry to seniority breaking effects

Highlighted the role of proton and neutron core excitations

Abstract

We performed \textit{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: $^{92}$Mo, $^{94}$Ru, $^{96}$Pd, and $^{98}$Cd. Our calculations well reproduced the measured low-lying spectra and electromagnetic $E2$ transitions in these nuclei, supporting partial seniority conservation in the first $\pi g_{9/2}$ shell. Recent experiments have revealed that, compared to the symmetric patterns predicted under the conserved seniority symmetry, the $4^+_1\to2^+_1$ $E2$ transition strength in $^{94}$Ru is significantly enhanced and that in $^{96}$Pd is suppressed. In contrast, the $6^+_1\to 4^+_1$ and $8^+_1\to6^+_1$ 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 \textit{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.