Ultracold heavy molecular anions as enhanced probes of <i>CP</i> violation

Abstract

Heavy polar molecules play a central role in precision measurements that probe physics beyond the Standard Model, most notably searches for the electron electric dipole moment (eEDM). Recent demonstrations of evaporative cooling of molecular anions have created an unexplored opportunity for extending such measurements to a new class of chemical species. Here, we present the comprehensive relativistic electronic structure investigation of a heavy molecular anion FrF−, which is anticipated to be a promising candidate for laser cooling and for studies of conjugation and parity violation. Using high-level two-component configuration–interaction methods, we carried out a detailed investigation of the cooling cycle mechanism of the FrF− molecular anion and evaluated the key parameters—effective electric field (Eeff), hyperfine constant (A‖), electron–nucleon scalar–pseudoscalar constant (WP,T), and nuclear quadrupole moment constant (WM). By quantitatively disentangling the contributions from basis-set hierarchy, electron correlation, spin–orbit coupling, and quantum electrodynamics effects, we provide a transparent and reliable uncertainty analysis for each constant. Our results reveal that FrF− exhibits a remarkably large intrinsic sensitivity to the eEDM, rivaling leading neutral and cationic candidates. These findings establish ultracold molecular anions as a powerful and previously untapped platform for next-generation symmetry tests and precision measurement science.