Mass Measurement of Upper fp -Shell N=Z-2 and N=Z-1 Nuclei and the Importance of Three-Nucleon Force along the N=Z Line

Abstract

Using a novel method of isochronous mass spectrometry, the masses of Ge62, As64, Se66, and Kr70 are measured for the first time, and the masses of Zn58, Ga61, Ge63, As65, Se67, Kr71, and Sr75 are redetermined with improved accuracy. The new masses allow us to derive residual proton-neutron interactions (δVpn) 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 δVpn 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 δVpn in even-even and odd-odd nuclei.