In a significant study published on October 23 in the journal Science, a team of researchers has made remarkable advancements in our understanding of atomic structure through the precise measurement of electrons orbiting a radium atom that is chemically bonded to a fluoride atom, forming radium monofluoride. This innovative approach transforms the molecular environment into a microscopic stand-in for a particle collider, allowing the electrons of the radium atom to be confined and increasing the probability that some of them briefly pass through the nucleus.
Traditional experiments that investigate nuclear interiors typically rely on kilometer-scale accelerators that propel electron beams to collide with and fragment atomic nuclei. However, the new molecule-centered approach provides a compact and efficient, table-top method to directly probe the internal structure of a nucleus. By working with radium monofluoride, the researchers meticulously tracked the energies of the radium atom's electrons as they moved within the molecule. They detected a small shift in energy, leading to the conclusion that some electrons must have briefly entered the nucleus and interacted with its contents. As these electrons exited the nucleus, they carried with them an energy change, effectively conveying a nuclear message that reveals intricate features of the nucleus's interior.
The implications of this research are profound, as the method paves the way for measuring the nuclear magnetic distribution. Within a nucleus, each proton and neutron behaves like a tiny magnet, with their orientations influenced by how these particles are arranged. The research team intends to utilize this technique to map these properties in radium for the first time, potentially shedding light on one of cosmology's most perplexing questions: why does the universe consist of far more matter than antimatter?
According to study co-author Ronald Fernando Garcia Ruiz, who serves as the Thomas A. Franck Associate Professor of Physics at MIT, "Our results lay the groundwork for subsequent studies aiming to measure violations of fundamental symmetries at the nuclear level. This could provide answers to some of the most pressing questions in modern physics." The MIT team includes co-authors Shane Wilkins, Silviu-Marian Udrescu, and Alex Brinson, along with collaborators from various institutions, including the Collinear Resonance Ionization Spectroscopy Experiment (CRIS) at CERN in Switzerland, where the experiments were conducted.
Current theories posit that the early universe should have contained nearly equal quantities of matter and antimatter. However, nearly all detectable matter today is composed of protons and neutrons within atomic nuclei, which contradicts expectations derived from the Standard Model. This discrepancy suggests that additional sources of fundamental symmetry violation are necessary to explain the scarcity of antimatter. Such effects are believed to manifest within the nuclei of specific atoms, including radium.
Unlike most atomic nuclei that are roughly spherical, radium's nucleus exhibits an asymmetric, pear-like shape. Theoretical predictions indicate that this unique geometry could amplify signals of symmetry violation, making them potentially observable. Garcia Ruiz emphasizes the significance of the radium nucleus, stating, "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."
Investigating the internal structure of a radium nucleus to test fundamental symmetries presents considerable challenges. Radium is inherently radioactive and has a short lifespan, and currently, only minute quantities of radium monofluoride molecules can be produced. Study lead author Shane Wilkins, a former postdoc at MIT, notes, "We therefore need incredibly sensitive techniques to be able to measure them." The researchers recognized that embedding a radium atom within a molecule could confine and magnify the behavior of its electrons, enhancing the chances of interaction with the nucleus.
By incorporating the radioactive atom into a molecule, the internal electric field experienced by its electrons becomes orders of magnitude larger than those generated in a laboratory setting. Co-author Silviu-Marian Udrescu explains, "In a way, the molecule acts like a giant particle collider and gives us a better chance to probe the radium's nucleus."
The research team successfully created radium monofluoride by pairing radium atoms with fluoride atoms. In this molecular configuration, the electrons of radium are effectively compressed, thereby increasing the likelihood of interaction with the nucleus. The researchers proceeded to trap and cool the molecules, guiding them through vacuum chambers while illuminating them with lasers designed to interact with the molecules. This meticulous setup facilitated precise measurements of electron energies within each molecule.
The measured energies exhibited a subtle deviation 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 for excitation, it provided compelling evidence that the electrons interacted with protons and neutrons inside the radium nucleus. Wilkins elaborates, "When we measured these electron energies very precisely, it didn't quite add up to what we expected, assuming they interacted only outside of the nucleus. That discrepancy indicated that the difference must result from electron interactions inside the nucleus." Garcia Ruiz adds, "We now have proof that we can sample inside the nucleus—it’s akin to measuring a battery's electric field from within."
Looking ahead, the research team aims to apply their novel technique to map the distribution of forces within the nucleus. Thus far, their experiments have involved radium nuclei in random orientations at elevated temperatures. Garcia Ruiz and his collaborators aspire to cool these molecules and control the orientations of their pear-shaped nuclei, allowing for precise content mapping and the exploration of fundamental symmetry violations.
According to Garcia Ruiz, "Radium-containing molecules are predicted to be exceptionally sensitive systems in which to search for violations of the fundamental symmetries of nature. We now have a way to carry out that search." This pioneering research was partially funded by the U.S. Department of Energy, and it represents a crucial step forward in our quest to understand the fundamental building blocks of the universe.