Precision Measurements of Radioactive Molecules for Fundamental Physics

The Science

For the first time, nuclear physicists made precision measurements of a short-lived radioactive molecule, radium monofluoride (RaF). The researchers combined ion-trapping techniques with specialized laser systems to measure the fine details of the quantum structure of RaF. This allowed the characterization of the rotational energy levels of this molecule as well as the determination of its laser-cooling scheme. Laser cooling is a method that uses laser light to slow down and trap atoms and molecules. These results represent a pivotal step for future experiments aiming to laser-cool and trap RaF molecules.

The Impact

Scientists predicted that molecules that contain heavy, pear-shaped nuclei, such as radium, are highly sensitive to nuclear electroweak properties and physics beyond the Standard Model. This includes phenomena that violate parity and time-reversal symmetry. Time reversal-violation, beyond the current constrains, is an essential condition to explain the matter-antimatter asymmetry of the universe. The new results give researchers a detailed characterization of the quantum structure of RaF, opening the use of this molecule in future experiments aiming to search for such effects.

Summary

Radioactive molecules containing octupole deformed nuclei, such as radium (Ra), promise to be exceptional quantum systems for use in studies of the fundamental particles and forces of nature. The unique pear-like shape of the radium nucleus, combined with the energy level structure of a polar molecule, can lead to an enhanced sensitivity to symmetry-violating nuclear properties of more than five orders of magnitude compared to stable atoms. Recently, nuclear physicists at Massachusetts Institute of Technology and collaborators investigated spectroscopically, for the first time, the detailed structure of radium monofluoride (RaF). They performed the work at the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at the Isotope Separator On Line Device Radioactive Ion Beam Facility at the European Organization for Nuclear Research (ISOLDE – CERN).

The researchers’ method allowed the mapping, with high sensitivity, of the energy levels of RaF, determining a laser cooling scheme for slowing and trapping this molecule. Scientists are rapidly developing methods of controlling and interrogating ultra-cold molecules. These methods, combined with the new capabilities of radioactive beam facilities to produce large amounts of radioactive molecules, such as CERN (Switzerland) and FRIB (US), are opening a new frontier in the exploration of atomic nuclei and the violation of the fundamental symmetries of nature.

Funding

This work was supported by the U.S. Department of Energy Office of Science, the Office of Nuclear Physics; the MISTI Global Seed Funds; Deutsche Forschungsgemeinschaft (DFG, German Research Foundation); Belgian Excellence of Science (EOS); KU Leuven C1 project; International Research Infrastructures (IRI) project; the European Unions Grant Agreement (ENSAR2); LISA: European Union’s H2020 Framework Programme; and the Swedish Research Council.