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Deep within atoms, scientists are searching for clues to some of the universe’s greatest mysteries. Recent studies show that extremely precise measurements of atomic properties can act as powerful microscopes for new physics, potentially revealing particles and forces that have never been observed.
Researchers from the Max Planck Institute for Nuclear Physics and Heidelberg University are exploring how atomic experiments can test ideas beyond the Standard Model of particle physics. While this theory successfully describes known particles and forces, it cannot explain phenomena such as dark matter. Many theories therefore propose new lightweight particles that could mediate an additional interaction, sometimes called a “fifth force”.
Searching for invisible particles
In the zoo of particles, bosons are the "messengers" that tell matter how to behave. Think of them as the different types of signals or ripples sent across the fields that fill our universe. They may come in three shapes:
(i) Scalar bosons: imagine a field that is perfectly uniform, like the temperature in a room or the pressure of the air. A scalar boson is a ripple in that kind of field. It looks exactly the same no matter which direction you view it from—much like a perfectly smooth billiard ball. It represents a simple value in space without any inherent "pointing" direction.
(ii) Pseudoscalar bosons: these are the mirror-image twins of the scalars. Like their cousins, they have no direction, but they possess a unique "spatial flip". If you were to look at such a pseudoscalar boson in a mirror, its mathematical properties would reverse or flip sign. It’s the subatomic version of a left-handed glove that appears to be a right-handed glove when reflected.
(iii) Vector bosons: these are the heavy lifters of force. They have an inherent angular momentum, a so-called spin, which gives them an orientation—think of them as tiny arrows pointing through space. Because they have "direction," they are the perfect messengers for carrying forces between particles. An example for a vector boson is the photon which carries light and electromagnetism. They don’t just exist at a point; they actively "push" or "pull" in a specific direction.
Among the new bosons proposed to fix the Standard Model are axion-like particles and new types of vector bosons. If such particles exist, they could subtly change the way electrons interact with the nuclei of atoms. Although these forces would be extremely weak, they could leave detectable traces in precise atomic measurements. Two particularly sensitive quantities are: (i) hyperfine splitting, a tiny difference in atomic energy levels caused by magnetic interactions between the nucleus and its electrons, and (ii) the electron g-factor, which describes how strongly an electron responds to a magnetic field.
The electron’s magnetic fingerprint
An electron behaves like a tiny bar magnet because of its intrinsic angular momentum, also called spin. The strength of this magnetic behaviour is described by the g-factor, which links the electron’s spin and magnetic moment. Quantum effects slightly modify this value, and theoretical physicists can calculate and experimentalists can measure it with extraordinary precision. If a new particle interacted with electrons or with the nucleus, it could change the measured value by a tiny amount. One powerful testing ground is hydrogen-like ions, atoms stripped down to a single electron. In these systems, the electron experiences very strong electric fields from the nucleus, which enhances subtle physical effects.
Removing nuclear uncertainty
Detecting new physics in atoms is challenging because atomic nuclei are complex. Their internal structure creates additional effects that can obscure possible signals from new particles. To overcome this problem, researchers compare carefully chosen atomic systems so that unwanted nuclear contributions cancel out. One method compares hydrogen-like ions (with one electron) and lithium-like ions (with three electrons) [1]. “By forming a weighted difference between their hyperfine splittings, most nuclear uncertainties disappear, leaving a signal that is much more sensitive to new forces”, explains PhD student Cedric Quint. Measurements of beryllium ions already set strong limits, while future experiments with heavier ions such as caesium could further increase sensitivity to axion-like particles and new vector bosons.
A second strategy focuses on the electron’s g-factor. Instead of comparing electronic states, researchers compare ions with different numbers of protons and neutrons. This approach, called a nuclide shift, cancels certain interactions and isolates the effect of potential new forces between electrons and protons. Using existing experimental data and theoretical calculations, the researchers determined how strongly hypothetical particles could interact with matter. The results significantly tighten existing limits. “In some cases, the new techniques improve constraints on possible electron–proton interactions by up to three orders of magnitude”, according to Matteo Moretti, the first author of the other article [2], with both from the Keitel division.
A new role for atomic physics
These studies show that cutting-edge atomic experiments are becoming powerful tools for exploring fundamental physics. In addition to relying on massive particle accelerators, researchers can use extremely precise measurements of single trapped ions or electrons to search for new particles. So far, these elusive “ghost particles” remain undiscovered. But each improvement narrows the range of possible theories and brings physicists closer to understanding the deeper laws that govern the universe.
Original publications:
[1] Stringent Constraints on New Pseudoscalar and Vector Bosons from Precision Hyperfine Splitting Measurements
Cedric Quint, Fabian Heiße, Joerg Jaeckel, Lutz Leimenstoll, Christoph H. Keitel and Zoltán Harman
Phys. Rev. Lett. 136 (2026) 113001, DOI: https://doi.org/10.1103/rvt1-93v2
[2] Fermion-Selective Tests of New Physics with the Bound-Electron g Factor
M. Moretti, C. H. Keitel, and Z. Harman
Phys. Rev. Lett. 136 (2026) 011803, DOI: https://doi.org/10.1103/zyb6-lvy8
Weblinks:
Division ‘Theoretical Quantum Dynamics and Quantum Electrodynamics’ at MPIK
Prof. Joerg Jaeckel, Institute for Theoretical Physics, Heidelberg University g
