Gamow-Teller transitions and neutron-proton-pair transfer reactions

We propose a schematic model of nucleons moving in spin--orbit partner levels, $j=l\pm{\sfrac12}$, to explain Gamow--Teller and two-nucleon transfer data in $N=Z$ nuclei above $^{40}$Ca. Use of the $LS$ coupling scheme provides a more transparent approach to interpret the structure and reaction data.We apply the model to the analysis of charge-exchange, $^{42}$Ca($^3$He,t)$^{42}$Sc, and np-transfer, $^{40}$Ca($^3$He,p)$^{42}$Sc, reactions data to define the elementary modes of excitation in terms of both isovector and isoscalar pairs, whose properties can be determined by adjusting the parameters of the model (spin--orbit splitting, isovector pairing strength and quadrupole matrix element) to the available data. The overall agreement with experiment suggests that the approach captures the main physics ingredients and provides the basis for a boson approximation that can be extended to heavier nuclei. Our analysis also reveals that the SU(4)-symmetry limit is not realized in $^{42}$Sc.