Role of momentum transfer in the quenching of Gamow-Teller strength

The relativistic proton-neutron quasiparticle random phase approximation (pn-RQRPA) is applied in the calculation of the L=0 strength in charge-exchange reactions on $^{48}$Ca, $^{90}$Zr, $^{208}$Pb and nuclei in the Sn isotopic chain. The microscopic theoretical framework is based on the relativistic Hartree-Bogoliubov (RHB) model for the nuclear ground state. The calculation is fully self-consistent, i.e. the same interaction is used both in the RHB equations that determine the quasiparticle basis, and in the matrix equations of the pn-RQRPA. The inclusion of the higher-order terms that include the effect of finite momentum transfer, primarily the isovector spin monopole (IVSM) term, in the transition operator shifts a portion of the strength to the high-energy region above the Gamow-Teller (GT) resonance. The total strength is slightly enhanced in nuclei with small neutron-to-proton ratio but remains unchanged with increasing neutron excess. Based on the strength obtained using the full L=0 transition operator in the pn-RQRPA calculation, we have estimated the impact of the IVSM on the strength measured in the charge-exchagne reactions on $^{90}$Zr and found that the data are consistent with the Ikeda sum rule.