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

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 $π{0f_{7/2}}$ 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$\%$.