According to a study published in Physical Review Letters, scientists are utilizing trapped ions in tests to look for evidence of a new particle that might help explain the enigmatic dark matter. Researchers at ETH Zurich are integrating their findings with those of teams in Germany and Australia.
Luca Huber, doctoral student at ETH Zurich, aligns a laser light for an experiment with calcium isotopes. Image Credit: Gabriele Giuli / ETH Zurich
The Standard Model of particle physics provides an accurate description of the fundamental forces between elementary particles, as well as the basic building blocks that make up all matter.
The Standard Model is currently the best explanation of the universe, but we know it cannot explain everything.
Diana Prado Lopes Aude Craik, Professor, ETH Zurich
Diana Prado Lopes Aude Craik cites dark matter as “one of the biggest mysteries in physics today”.
The rotation of galaxies cannot be explained by visible matter alone, according to astronomical data. Scientists are searching for a theory that extends beyond the Standard Model of particle physics, driven by the belief that most of the universe’s mass comes from an unknown type of matter.
Some intriguing ideas anticipate the presence of a new, fifth force of nature, in addition to the four fundamental forces: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.
For example, neutrons in the atomic nucleus are supposed to interact with electrons in the atom's shell via an unknown force. This force could be transmitted by a new particle, much as photons carry electromagnetic force.
Measuring the Atom with Precision
Researchers have long used particle accelerators, such as those at CERN in Geneva, to seek new particles that do not fit into the Standard Model. Aude Craik and her colleagues from Professor Jonathan Home’s research group at the ETH Institute of Quantum Electronics have a different method.
Craik added, “As atomic physicists, we can measure the atom with extremely high precision. Therefore, the idea is to search for this new force between the neutron and the electron using precision atomic spectroscopy.”
This experiment is being carried out by a team located in Zurich in collaboration with researchers in Germany and Australia.
If this force truly exists within the atom, then its strength is proportional to the number of neutrons in the atomic nucleus. That is why we are experimenting with isotopes to detect this hypothetical force.
Luca Huber, Doctoral Student, ETH Zurich
Isotopes are versions of the same element that differ only in the number of neutrons in their nuclei. This means they share the same number of protons and electrons, making them chemically similar, but they differ in mass. Because of this mass difference, the overall force acting on the electrons can vary slightly depending on the number of neutrons.
Scientists can detect these subtle shifts by examining the energy levels at which electrons move within the atom. If a new fundamental force exists, researchers expect it would cause slight changes in these energy levels across different isotopes.
Studying Calcium Isotopes in a Precision Ion Trap
“To determine these energy shifts, we measure the frequency of the light emitted when our isotopes transition between two energy levels,” explained Aude Craik.
This measurement needs an ion trap, in which electromagnetic fields hold a single charged isotope in place while a laser excites it to a higher energy state. The researchers employed five stable, singly charged calcium isotopes in their studies. Each isotope had 20 protons, while the number of neutrons varied from 20 to 28.
It is 100 times more accurate than the best prior measurements, and the researchers were able to determine the shifts in energy levels of these isotopes in the lab with an accuracy of 100 millihertz.
“We trapped two isotopes simultaneously in the ion trap and measured them together,” Huber explained.
They were able to significantly lower the interfering noise during the frequency measurement as a result.
Despite this accuracy, further tests were required to pursue the hunt for novel physics. While the Zurich team experimented with singly charged calcium isotopes, Piet Schmidt's research group at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig employed the same isotopes in a multiply charged condition.
The German group recorded a distinct transition in these highly charged calcium ions with similar precision to the Zurich team. A third group, led by Klaus Blaum of the Max Planck Institute for Nuclear Physics in Heidelberg, determined the nuclear mass ratios of these isotopes with exceptional accuracy.
More Precise Constraints Determined
Other research teams in Germany and Australia performed precise calculations to accurately understand this conclusion. Their findings demonstrate that well-understood nuclear effects account for just a portion of the divergence. Another proposed reason is nuclear polarisation, a sort of electron-induced distortion of the atomic nucleus that has received little attention yet.
Its complicated calculation demonstrates that nuclear polarisation might be significant enough to explain the observed nonlinearity within the limitations of the Standard Model.
Aude Craik added, “We cannot say that we have discovered new physics here. However, we know how strong the new force can be at most because we would have seen it otherwise in our measurements, even with the uncertainties.”
The researchers can now limit the potential possibilities for the mass and charge of the hypothetical particle that would convey the new force.
The researchers are now attempting to increase the accuracy of their results.
Huber stated, “We are presently measuring a third energy transition in the calcium isotopes, and doing so with even greater precision than before.”
Their goal is to turn the King plot from a two-dimensional diagram into a three-dimensional one.
“We hope that this will help us overcome the theoretical challenges and make further progress in the search for this new force,” Aude Craik concluded.
Journal Reference:
Wilzewski, A., et al. (2025) Nonlinear Calcium King Plot Constrains New Bosons and Nuclear Properties. Physical Review Letters. doi.org/10.1103/physrevlett.134.233002.