Physicists at MIT have developed a new method using molecules to investigate the interior of atomic nuclei, employing electrons as messengers in a tabletop setup. By studying radium monofluoride, they detected subtle energy shifts indicating electron interactions inside the nucleus. This approach could help explain the universe's matter-antimatter imbalance.
In a study published on October 23 in Science, researchers from MIT introduced a technique that turns molecules into microscopic particle colliders to glimpse inside atomic nuclei. They focused on radium monofluoride (RaF), where electrons orbiting the radium atom are confined, increasing the chances of briefly entering the nucleus. Traditional methods rely on massive, kilometer-scale accelerators to smash electron beams into nuclei, but this molecular approach offers a compact, tabletop alternative.
The experiments, conducted at the Collinear Resonance Ionization Spectroscopy Experiment (CRIS) at CERN in Switzerland, involved trapping and cooling RaF molecules, then illuminating them with lasers to measure electron energies precisely. The team observed a small energy shift—about one millionth of the laser photon's energy—suggesting electrons interacted with protons and neutrons inside the nucleus, carrying away a 'nuclear message' upon exiting.
"There are many experiments measuring interactions between nuclei and electrons outside the nucleus, and we know what those interactions look like," explained lead author Shane Wilkins, a former MIT postdoc. "When we went to measure these electron energies very precisely, it didn't quite add up to what we expected assuming they interacted only outside of the nucleus."
This breakthrough paves the way for mapping the nuclear magnetic distribution in radium, whose pear-shaped nucleus is predicted to amplify signals of fundamental symmetry violations. Such violations could account for why the universe favors matter over antimatter, contrary to the Standard Model's expectations.
"Our results lay the groundwork for subsequent studies aiming to measure violations of fundamental symmetries at the nuclear level," said co-author Ronald Fernando Garcia Ruiz, the Thomas A. Franck Associate Professor of Physics at MIT. "This could provide answers to some of the most pressing questions in modern physics."
The radium nucleus's asymmetric shape in charge and mass makes it uniquely suited for these probes, as noted by Garcia Ruiz: "The radium nucleus is predicted to be an amplifier of this symmetry breaking, because its nucleus is asymmetric in charge and mass, which is quite unusual."
Future plans include cooling the molecules to control nuclear orientations and hunt for symmetry violations more effectively. The research was supported in part by the U.S. Department of Energy, with MIT co-authors including Silviu-Marian Udrescu and Alex Brinson, alongside international collaborators.