Spin-triplet proton-neutron pair in spin-dipole excitations

TL;DR

This study explores how spin-triplet proton-neutron pairing influences low-lying spin-dipole transitions in nuclei, revealing its significant effect especially in high-angular-momentum orbitals, and clarifying its role distinct from Gamow-Teller transitions.

Contribution

It demonstrates the impact of $S=1$ pn-pair interactions on SD transitions using three-body and energy-density functional models, highlighting the importance of orbital angular momentum.

Findings

The effect depends on the spatial overlap of the pn pair.

Higher angular momentum orbitals show more significant effects.

SD transitions with same principal quantum number are strongly affected.

Abstract

Background: Spin-triplet ($S=1$) proton-neutron (pn) pairing in nuclei has been under debate. It is well known that the dynamical pairing affects the nuclear matrix element of the Gamow-Teller (GT) transition and the double beta decay. Purpose: We investigate the effect of the pn-pair interaction in the $T=0, S=1$ channel on the low-lying spin-dipole (SD) transition. We then aim at clarifying the distinction of the role in between the SD and GT transitions. Method: We perform a three-body model calculation for the transition ${}^{80}\mathrm{Ni}\to{}^{80}\mathrm{Cu}$, where ${}^{78}\mathrm{Ni}$ is taken as a core. The strength of the pair interaction is varied to see the effect on the SD transition-strength distribution. To fortify the finding obtained by the three-body model, we employ the nuclear energy-density functional method for the SD transitions in several nuclei, where one can expect a strong effect. Results: The effect of the $S=1$ pn-pair interaction depends on the spatial overlap of the pn pair and the angular momentum of the valence nucleons; the higher the angular momentum of the orbitals, the more significant the effect. Conclusions: The dynamical $S=1$ pairing is effective even for SD states although the spatial overlap of the pn pair can be smaller than GT states. The SD transition involving high-$\ell$ orbitals with the same principal quantum number is strongly affected by the dynamical $S=1$ pairing.