Breakthrough in muon research: Magnetic moment precision achieved!

Breakthrough in muon research: Magnetic moment precision achieved!

On June 3, 2025, the Muon g-2 collaboration presented its third and final measurement of the muon's anomalous magnetic moment. This new analysis arrived at an experimental value of aµ = (g−2)/2 = 0.001 165 920 705 ± 0.000 000 000 148 and exceeded the initial objectives with an unexpectedly high accuracy of 127 parts per billion. This demonstrates remarkable advances in precision measurement, which play an important role in modern particle physics.

The measurements were carried out at the Fermi National Accelerator Laboratory (Fermilab) and include data from a six-year research phase that lasted until July 9, 2023. During this period, over 308 billion muons were measured, with the accuracy of the measurement improving from 200 to 127 parts in a billion. Prof. Dr. Martin Fertl from Johannes Gutenberg University Mainz is the only German researcher in the international Muon g-2 collaboration, which brings together almost 180 scientists from 37 institutions in seven countries.

A look at the muon g-2 experiment

The Muon g-2 experiment tracks the precession of the magnetic moment of muons, which are similar to them but about 200 times heavier than electrons. These fundamental particles have relatively short lifetimes and extended properties that are affected by vacuum fluctuations. These fluctuations are also the reason for the current deviation of the anomalous magnetic moment, which deviates by about 0.1% from the theoretical value. The experiment uses a 14-meter-diameter superconducting magnetic ring to analyze the muons under controlled conditions.

The latest results are consistent with previous measurements from 2021 and 2023, but offered new, more precise data. The Muon g-2 Theory Initiative has simultaneously published new predictions for the anomalous magnetic moment, which give a theoretical value of aµ = (g−2)/2 = 0.001 165 920 33 ± 0.000 000 000 62 based on lattice QCD calculations. This agreement could provide evidence that there are physical phenomena that go beyond the Standard Model.

Connections to dark matter

Research into the anomalous magnetic moment could also provide important insights into dark matter, which is considered to be the basic building block for the structures in the universe. Physicists search for dark matter using two methods: through direct experiments at particle accelerators such as the Large Hadron Collider, and through indirect studies of known physical processes that require precision. The measurements at Fermilab have shown that muons are able to search the vacuum for virtual particles and thus potentially discover new particles that could make up dark matter.

The experiments at Fermilab have also revolutionized the understanding of theoretical calculations that in the past diverged from observations. In the most recent study, new insights emerged from more detailed consideration of vacuum fluctuations, making the deviations from the Standard Model more understandable.

Although the Muon g-2 experiment is now complete, it could still serve as a benchmark for future measurements. Another experiment in Japan is planned to provide additional data, albeit with lower precision, in the 2030s. The challenge is interpreting the results and finding answers to the questions raised by the latest results, including the need to clarify why no new particles were discovered in the LHC.

Recent revelations and developments in theoretical physics make it clear that the sometimes contradictory results of muon g-2 and the LHC experiments could lead to an exciting phase in particle physics in which old theories will be questioned and new ideas developed in order to better understand the universe and its fundamental forces.

Last but not least, the Muon g-2 collaboration defines the path for future scientific discoveries and advances the understanding of fundamental physical concepts that go beyond what we previously take for granted.

Further information and details about the study can be found here here, on the role of dark matter here and to discuss the search for new particles here.