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

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 Ni → 80 Cu, where 78 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-orbitals with the same principal quantum number is strongly affected by the dynamical S = 1 pairing.