A novel approach to the search for a long theorized fifth force of nature—one that could potentially help scientists unravel lingering mysteries involving the Standard Model of particle physics—is being undertaken by physicists in Switzerland.
Among these mysteries, the quest to resolve the elusive nature of dark matter, as well as the ongoing search for new particles and other discoveries that could herald the arrival of “new physics,” are at the heart of their investigations.
Although particle accelerators have led the charge in the hunt for such mysterious particles beyond the Standard Model, the research team at ETH Zurich has deployed a different tool to aid them in their search: precision atomic spectroscopy.
Analyzing very subtle shifts in energy levels between calcium isotopes, they hope, could help them detect signatures from an as-yet unknown force theorized to be acting between neutrons and electrons, which a previously undiscovered particle could carry.
Pushing the Boundaries on the Standard Model
Based on the Standard Model of particle physics, scientists can very precisely describe the building blocks that make up the matter around us, and even ourselves. It also offers a roadmap in terms of the behavior of the fundamental forces acting between these elementary particles.
Still, it has its limitations, a reality that ETH Zurich physics professor Diana Craik knows all too well.
“The Standard Model is currently the best explanation of the universe, but we know it cannot explain everything,” Craik says. The existence of dark matter, for instance, showcases the problems physicists like her encounter when it comes to conventional approaches to the study of our universe, since it can already be inferred from astronomical observations that the rotation of galaxies and other phenomena cannot be explained entirely based on the matter that can be seen.
Because of this, physicists like Craik have long believed that an unknown form of matter must be at play, a reality that will also require a theory that extends beyond the current Standard Model.
The Search for a Fifth Force
Fortunately, there are already a few promising theories that predict the existence of a new, fifth force of nature alongside the existing fundamental forces of gravity, electromagnetism, and the strong and weak nuclear forces. One tantalizing clue that could point to an undiscovered fifth force is based on observations of neutrons in the atomic nucleus and electrons present in the shell of atoms. Researchers believe that a force could be present that governs these subatomic behaviors—one that an undiscovered particle might carry.
Past efforts to discover new particles beyond the Standard Model have generally involved particle accelerators like the famous Large Hadron Collider (LHC) operated at CERN in Geneva, Switzerland. That isn’t to say that it’s the only approach that could lead to new physics discoveries.
“As atomic physicists, we can measure the atom with extremely high precision,” says Craik, a physicist with a research group led by Professor Jonathan Home at the ETH Institute of Quantum Electronics.
For physicists like Craik and her colleagues, “the idea is to search for this new force between the neutron and the electron using precision atomic spectroscopy.”
“If this force really exists in the atom, then its strength is proportional to the number of neutrons in the atomic nucleus,” said doctoral student Luca Huber, who is also part of the research team. “That’s why we are experimenting with isotopes to detect this hypothetical force.”
Isotopes and Ion Traps
Atoms of the same type that differ in terms of the number of neutrons in their atomic nucleus are known as isotopes, which can have the same number of protons and electrons, meaning they are chemically the same while possessing different masses.
Because of this, the combined force the electrons are subjected to varies slightly across different isotopes of the same atomic type due to variances in the number of neutrons. This offers researchers like those in Home’s team at ETH Zurich a novel means of potentially exploring such phenomena, since energy signatures arising from the movement of electrons within atoms are measurable.
In theory, slight changes in energy signatures produced between different isotopes should be discernible, which would likely be caused by the currently unknown fifth force that Home’s team is aiming to discover.
Craik says the team plans to unveil these suspected energetic changes by taking measurements of the light frequencies produced as isotopes undergo transitions between energy levels. Achieving this requires an “ion trap” in which an electromagnetic field is used to keep a single charged isotope in position while it is excited to higher energy states using a laser.
In their research, the Zurich team chose five stable, singly-charged calcium isotopes containing 20 protons, but with varying numbers of neutrons. During lab experiments, they successfully discerned shifts in energy levels from these isotopes down to an accuracy of 100 millihertz, around 100 times greater precision than past measurements had attained of such phenomena.
To achieve this, the team took a novel approach by trapping not one, but a pair of isotopes within their ion trap, collecting measurements simultaneously and limiting the interference that normally occurs during such measurements.
Greater Precision Atomic Spectroscopy
Even the unprecedented precision of their measurements required further experiments, where the Zurich team began to study calcium isotopes with single charges that were employed in a multiply charged state. The team’s collaborators in Germany and at the Max Planck Institute for Nuclear Physics in Heidelberg contributed by performing ratio measurements of the masses of nuclei between these isotopes, which further enhanced the precision of the team’s data.
Following these measurements, additional work was undertaken in Germany and Australia, which determined that the deviations the team observed in their high-precision measurements could only be partially explained through well-known nuclear effects. In other words, something more appears to be at work here.
An additional factor the team considered involves nuclear polarization, which involves a little-understood phenomenon where atomic nuclei undergo deformation caused by electrons. The calculations by the German and Australian teams appeared to show that nuclear polarization may be able to explain the measurements, albeit potentially also within the current framework of the Standard Model.
“We can’t say that we’ve discovered new physics here,” Craik said in a statement. “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 Ongoing Search for Nature’s Fifth Force
Going forward, the team now hopes to continue studying whether values for the mass and charge associated with a hypothetical particle could be determined, thereby helping them determine the values that would be required for the existence of its associated new force.
“We are currently measuring a third energy transition in the calcium isotopes,” Huber said of the team’s experiments, adding that she and her colleagues aim to obtain even greater precision measurements than their past studies achieved.
“We hope that this will help us overcome the theoretical challenges,” Craik added, “and make further progress in the search for this new force.”
The team’s groundbreaking work was recently detailed in a new paper, “Nonlinear Calcium King Plot Constrains New Bosons and Nuclear Properties,” published in Physical Review Letters on June 10, 2025, and in a feature at ETH Zurich’s site by Barbara Vonarburg.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.
