In research published on October 23 inScience, the team precisely measured the energy of electrons orbiting a radium atom that was chemically bound to a fluoride atom, forming radium monofluoride. By using the molecular environment as a microscopic stand-in for a particle collider, they confined the radium atom's electrons and increased the likelihood that some would briefly pass through the nucleus.

Traditional experiments that investigate nuclear interiors depend on kilometer-scale accelerators that speed up electron beams to smash into and fragment nuclei. The new molecule-centered approach provides a compact, table-top way to directly probe the inside of a nucleus.

Table-Top Method Detects Nuclear "Messages"

Working with radium monofluoride, the researchers tracked the energies of the radium atom's electrons as they moved within the molecule. They observed a small shift in energy and concluded that some electrons must have briefly entered the nucleus and interacted with what lies inside. As those electrons left, they retained the energy change, effectively carrying a nuclear "message" that reveals features of the nucleus's interior.

The method opens a path to measuring the nuclear "magnetic distribution." Inside a nucleus, each proton and neutron behaves like a tiny magnet, and their orientations depend on how these particles are arranged. The team plans to use the technique to map this property in radium for the first time, a step that could inform one of cosmology's central puzzles: why the universe contains far more matter than antimatter.

"Our results lay the groundwork for subsequent studies aiming to measure violations of fundamental symmetries at the nuclear level," says study co-author Ronald Fernando Garcia Ruiz, who is the Thomas A. Franck Associate Professor of Physics at MIT. "This could provide answers to some of the most pressing questions in modern physics."

MIT co-authors include Shane Wilkins, Silviu-Marian Udrescu, and Alex Brinson, together with collaborators from several institutions, including the Collinear Resonance Ionization Spectroscopy Experiment (CRIS) at CERN in Switzerland, where the experiments took place.

Matter-Antimatter Imbalance and Radium's Role

According to current understanding, the early universe should have contained nearly equal amounts of matter and antimatter. Yet nearly everything we can detect today is matter built from protons and neutrons inside atomic nuclei.

This observation conflicts with expectations from the Standard Model, suggesting that additional sources of fundamental symmetry violation are needed to account for the scarcity of antimatter. Such effects could appear within the nuclei of certain atoms, including radium.

Unlike most nuclei, which are close to spherical, radium's nucleus has an asymmetric, pear-like shape. Theorists predict that this geometry can amplify signals of symmetry violation enough to make them potentially observable.

"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," says Garcia Ruiz, whose group has focused on developing methods to probe radium nuclei for signs of fundamental symmetry violation.

Building Ultra-Sensitive Molecular Experiments Peering inside a radium nucleus to test fundamental symmetries is extremely challenging.

"Radium is naturally radioactive, with a short lifetime and we can currently only produce radium monofluoride molecules in tiny quantities," says study lead author Shane Wilkins, a former postdoc at MIT. "We therefore need incredibly sensitive techniques to be able measure them."

The team recognized that embedding a radium atom in a molecule could confine and magnify the behavior of its electrons.

"When you put this radioactive atom inside of a molecule, the internal electric field that its electrons experience is orders of magnitude larger compared to the fields we can produce and apply in a lab," explains Silviu-Marian Udrescu PhD '24, a study co-author. "In a way, the molecule acts like a giant particle collider and gives us a better chance to probe the radium's nucleus."

Energy Shift Reveals Electron-Nucleus Encounters

The researchers created radium monofluoride by pairing radium atoms with fluoride atoms. In this molecule, the radium electrons are effectively squeezed, which increases the chance that they will interact with and briefly enter the radium nucleus.

They then trapped and cooled the molecules, guided them through vacuum chambers, and illuminated them with lasers tailored to interact with the molecules. This setup allowed precise measurements of electron energies inside each molecule.

The measured energies showed a subtle difference from expectations based on electrons that do not enter the nucleus. Although the energy change was only about one millionth of the energy of the laser photon used to excite the molecules, it provided clear evidence that the electrons interacted with protons and neutrons inside the radium nucleus.

"There are many experiments measuring interactions between nuclei and electrons outside the nucleus, and we know what those interactions look like," Wilkins explains. "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. That told us the difference must be due to electron interactions inside the nucleus."

"We now have proof that we can sample inside the nucleus," Garcia Ruiz says. "It's like being able to measure a battery's electric field. People can measure its field outside, but to measure inside the battery is far more challenging. And that's what we can do now."

Next Steps: Mapping Forces and Testing Symmetries

Going forward, the team plans to apply the new technique to map the distribution of forces inside the nucleus. Their experiments have so far involved radium nuclei that sit in random orientations inside each molecule at high temperature. Garcia Ruiz and his collaborators would like to be able to cool these molecules and control the orientations of their pear-shaped nuclei such that they can precisely map their contents and hunt for the violation of fundamental symmetries.

"Radium-containing molecules are predicted to be exceptionally sensitive systems in which to search for violations of the fundamental symmetries of nature," Garcia Ruiz says. "We now have a way to carry out that search."

This research was supported, in part, by the U.S. Department of Energy.

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