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At 10:00 a.m. CDT on June 3, 2025 (early June 4 JST), Fermilab (Fermi National Accelerator Laboratory) released the final experimental results of the Muon g-2 experiment, which investigates the anomalous magnetic moment of the muon. The official announcement can be found at: https://muon-g-2.fnal.gov/

The anomalous magnetic moment refers to the deviation in the magnetic strength of a fundamental particle that arises due to quantum effects. While this quantity can be calculated with high precision based on the Standard Model of particle physics, the presence of unknown particles or forces could be indicated by measurable discrepancies between theoretical predictions and experimental results. In the case of the muon g-2, a significant gap between theory and experiment has persisted for over two decades. Physicists around the world have been closely watching the Muon g-2 experiment, as precise measurements could provide hints of “new physics” beyond the Standard Model.

Fermilab began the Muon g-2 experiment in 2018 and released intermediate results in 2021 and 2023. Data acquisition was completed in 2023, and the final results announced this time are based on the full dataset collected throughout the project. With a precision of 127 parts per billion (ppb)—equivalent to an uncertainty in the 7th to 8th decimal place—this measurement determines the value of the muon g-2 with unprecedented accuracy, achieving the highest precision in the world.
However, as will be mentioned later, theoretical predictions are not yet fully consistent, and therefore the muon g-2 measurement cannot be conclusively attributed to new physics. Further progress in both theoretical and experimental research is expected.

Press release from Fermilab:
Muon g-2 announces most precise measurement of the magnetic anomaly of the muon

The latest theoretical prediction of muon g-2 within the Standard Model has been published in the updated version of the “White Paper,” released on May 28, 2025  (https://arxiv.org/abs/2505.21476).

In the previous White Paper, published in 2020, the Standard Model prediction was significantly lower than the experimental measurement, which drew considerable attention. In the newly released version, the situation is presented in terms of two theoretical approaches: the data-driven estimate based on electron-positron annihilation data, and the lattice QCD calculation, which shows no significant discrepancy with the experimental value. The White Paper explains that it is difficult to simultaneously reconcile both theoretical evaluations without contradiction.

In this update, due to the improved precision of lattice QCD calculations and the agreement among results from independent lattice QCD groups, the lattice-based value has been adopted as the representative prediction. To compare theory with experiment, it is essential to understand the difference between the two theoretical approaches — a major challenge that remains for the future. In this context, the Belle II experiment using SuperKEKB accelerator, located on the Tsukuba campus of KEK, is expected to provide new input for data-driven evaluations based on e⁺e⁻ collision data. The Muon g–2 Theory Initiative, a theoretical research community, aims to overcome these challenges and ultimately improve the precision of the Standard Model prediction by a factor of two.

At the Institute of Particle and Nuclear Studies, KEK, preparations are underway for a new muon g-2/EDM* experiment at the J-PARC facility in Tokai, Ibaraki Prefecture, Japan.

This experiment utilizes a world-first technique to “cool” muons—slowing them down and aligning their directions—before re-accelerating them to produce an extremely focused, low-emittance beam similar to a laser. This method allows ultra-precise measurements using a compact experimental setup only one-twentieth the size of the one used at Fermilab. Since this approach totally differs from Fermilab’s, it offers a valuable independent verification of the results. In 2024, the experimental team successfully demonstrated muon cooling and acceleration. The construction of the experimental facility is also progressing steadily, and in April 2025, the first beam was successfully extracted into the experimental area. The next phase involves step-by-step installation of the accelerator and experimental apparatus in this new facility.

 

 

Following the final experimental results from the Fermilab Muon g–2 experiment, Professor Liang Li of Shanghai Jiao Tong University, scheduled to give a seminar at KEK in late June on behalf of the Fermilab g–2 collaboration, and Professor Tsutomu Mibe of KEK’s Institute of Particle and Nuclear Studies, spokesperson for the J-PARC muon g–2/EDM experiment, shared their comments.

 

[Comment from Professor Liang Li]

“This result is truly a landmark measurement—one that will remain in textbooks for decades to come. It represents the most precise measurement of the muon anomalous magnetic moment and stands among the finest achievements in the history of science. With a final precision of 0.12 ppm, the Fermilab Muon g-2 experiment brings the storage-ring technique to its peak performance; surpassing this accuracy with the same method and apparatus will be extraordinarily difficult.

Yet the story of muon g-2 is far from over. Alternative approaches—whether at J-PARC, MUonE, or future muon facilities—promise new ways to probe this fundamental quantity. Intriguing discrepancies surrounding the muon g-2 persist. Today’s milestone will inspire the next generation of measurements. The quest to fully understand the muon’s magnetic anomaly continues.”

 

[Comment from Professor Tsutomu Mibe]

This is an outstanding measurement that surpasses the originally proposed precision. At J-PARC, we aim to rigorously verify this result using a new approach that employs accelerated muons and a compact storage magnet. In addition, we are planning a wide range of studies using accelerated muons, and we look forward to sharing further progress in the near future.

 

※EDM (Electric Dipole Moment):

Subatomic particles hold not only magnetic moment but also an electrical property called Electric Dipole Moment (EDM). When there is an imbalance of charge within a particle (or when the particle lacks spherical symmetry), its EDM becomes nonzero. However, according to the Standard Model, this effect is extremely small. The discovery of EDM would provide evidence of violation of time-reversal symmetry. In the muon g-2/EDM experiment at J-PARC, instruments will be suitably set up for measuring EDM, allowing for a more sensitive search for EDM. ( https://www2.kek.jp/ipns/en/news/4886/ )