2N and 3N Tensor Force in the $N=34$ Shell Evolution: An Ab Initio Perspective

TL;DR

This study uses ab initio methods to analyze how tensor components of nucleon interactions influence the evolution of the N=34 shell gap in calcium and nickel isotopes.

Contribution

It provides a detailed ab initio analysis of the tensor-force contributions to shell evolution, emphasizing the roles of NN and 3N forces in N=34 shell gap changes.

Findings

The N=34 shell gap decreases with increasing proton occupancy in the f7/2 orbital.

Tensor forces, especially NN tensor force, predominantly drive the shell gap disappearance.

Approximately 83% of the shell gap reduction is due to NN tensor force, with 17% from 3N tensor force.

Abstract

Shell evolution plays a vital role in understanding the nuclear shell structures across the nuclear chart. In this work, we have investigated the $N = 34$ shell structure using the state-of-the-art ab-initio valence-space in-medium similarity renormalization (VS-IMSRG) approach. Notably, we employ nucleon-nucleon (NN) and three-nucleon (3N) interactions derived from chiral effective field theory and make use of the spin-tensor decomposition scheme to examine the contributions of individual interaction components. We discuss the evolution of the shell structures, which have been investigated by considering the roles of various components, including central, spin-orbit, and tensor effects of NN and 3N forces, respectively. The $N=34$ shell gap gradually decreases from $^{54}$Ca as the proton occupancy in the $\pi{0f_{7/2}}$ orbital increases, and eventually disappears in the $^{62}$Ni as a consequence of the tensor-force driven shell evolution. Our analysis reveals that this disappearance is predominantly governed by the NN tensor force, which accounts for approximately 83$\%$, while the 3N tensor force also contributes about 17$\%$.