Neutron-Proton, Neutron-Neutron, Proton-Proton QRPA for the Gamow-Teller and the M1 Spin Transitions of N ≃ Z Nuclei in the s — d Shell

We study the Gamow-Teller (GT) and the M1 spin transitions of the s — d shell nuclei 24Mg, 26Mg, 28Si, 32S, and 36Ar by using a quasiparticle random phase approximation (QRPA) scheme that comprises neutron-proton (np), neutron-neutron (nn) and proton-proton (pp) QRPA owing to the inclusion of the np pairing, as well as nn and pp pairing correlations. At present, we work in spherical symmetry under the assumption that the deformations of the nuclei considered in this work are small enough to neglect, as was customary in past applications. For the GT transition, as well as single and double beta decays, the neutron-proton QRPA (np QRPA) is usually employed. For the M1 spin transition, the pp and nn QRPA is needed due to its neutral current property. The np pairing correlations couple the np QRPA to the nn and pp QRPA. Therefore, the coupled np + nn + pp QRPA enables us to investigate simultaneously the GT and the M1 spin transitions. In particular, in this work, detailed analyses of particle-particle and particle-hole interactions at the QRPA stage are performed for the GT and the M1 spin transitions of the well-known s — d shell N ≃ Z nuclei, and the results are compared to available data. The roles of the np pairing correlations are also discussed for those transitions.