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 (E-eff), S-PS coefficient (W-s), 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 E-eff, W-s, 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 E-eff and W-s, 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 (E-pol).