Merits of heavy-heavy diatomic molecules for electron electric-dipole-moment searches

The electric dipole moment of the electron (eEDM) and the scalar-pseudoscalar (S-PS) interaction are probes of new physics beyond the standard model of elementary particles, but experiments to observe them using atoms and molecules are still in progress. Molecules that have a large effective electric field (Eeff), S-PS coefficient (Wₛ), and permanent electric dipole moment (PDM) are in principle favorable candidates for such experiments, and hence, it is necessary to analyze these properties. In this work, we calculate Eeff, Wₛ, and PDM for Ra systems: RaF, RaX (X=Cl, Br, I, and At), and RaZ (Z=Cu, Ag, and Au), using the Dirac-Fock and the relativistic coupled-cluster methods. The size of Eeff and Wₛ, and Ra of RaF, RaX, and RaZ are overall similar, but the values of RaX and RaZ (except for RaAu) are larger than those of RaF, up to about $40%$. We explain this finding by taking into consideration the large s-p mixing for RaX and RaZ, similar to what we had done in our previous work using hydrides and fluorides [Sunaga et al., Phys. Rev. A 95, 012502 (2017)]. We also discuss the suitability of RaX and RaZ molecules for eEDM experiments from the viewpoint of their large PDM and small polarizing electric field (Eₚₒₗ).