Muon Experiment Calls It a Wrap
After measuring the wobbles of 300 billion muons, the Muon g − 2 Collaboration has pinpointed with exquisite precision the internal magnetism of these subatomic particles [1]. The muon’s magnetic strength, or moment, has animated particle physics research over the past two decades, as experiment and theory appeared to disagree over its value—raising a flag for possible new physics. In a somewhat surprising turn of events, the final results from the Muon g − 2 experiment line up with the most recent predictions, further validating the standard model of particle physics.
The muon—the heavy cousin to the electron—started to grab the particle-physics spotlight in the 1990s when an experiment at Brookhaven National Laboratory in New York reported the first hints that the muon’s magnetic behavior might not match predictions based on the standard model, which has otherwise been widely successful in explaining the subatomic world. The Brookhaven measurements involved magnetically trapping muons in a circular ring and observing how much their internal magnet, or “spin,” wobbled around an applied magnetic field. To further investigate this discrepancy, the experiment’s big magnet was moved cross-country in 2013 to Fermi National Laboratory (Fermilab) in Illinois. The first results from the transplanted Muon g − 2 experiment came out in 2021, showing good agreement with the Brookhaven findings and raising the significance of the discrepancy (see Viewpoint: Muon’s Escalating Challenge to the Standard Model).
The Muon g − 2 Collaboration has now finished collecting data and released its final analysis [1]. As is commonly done, the muon moment is expressed in terms of the anomalous magnetic moment , which quantifies how far off the particle’s normalized moment, called g, is from a reference value of 2 (hence, the experiment name “g minus 2”). The team’s final value of = 0.001165920705 is squarely in-line with the previous, less-precise, experimental outcomes. “It was a big, big relief seeing the result come out right on top of our former results,” says the collaboration’s spokesperson Peter Winter from Argonne National Laboratory in Illinois. “It’s a confirmation that the hard work has really paid off.”
The new result stands out for its sensitivity, with error bars that are 4 times smaller than those of the Brookhaven experiment and 1.6 times smaller than those of the previous Fermilab experiment. The precision stands at 127 parts per billion, which Winter compares to weighing a bison to the nearest 100 mg (equivalent to a sunflower seed). “The experimental value stands now on very solid footing,” Winter says. “It will be hard to beat this precision in the future.”
“The Muon g − 2 result is an experimental tour de force,” says Priscilla Cushman, a physicist from the University of Minnesota who worked on the Brookhaven experiment. The experimental value for the magnetic moment has remained consistent, while the error bars have shrunk, restricting the wiggle room for theorists. “Any extension to the standard model will have to fit within these very strict bounds, creating a lasting benchmark for judging their credibility.”
The possibility that the muon results might point to new physics was diminished by inconsistencies that emerged on the theory side in the past few years. There are two main ways of calculating the magnetic moment of the muon, known as the data-driven method and the lattice QCD method. The former, which was the gold standard for many years, predicted a low value of the muon moment—below the value around which the experiment values were clustered. By contrast, results from the lattice method have been higher (see News Feature: Repeated Particle Measurements Disagree with Theory—What Now?).
The impasse between theory camps led to reassessments on both sides. Recent lattice results have increased the theory community’s confidence in that approach. On the other hand, discrepancies have appeared in the data-driven method, casting doubts on its validity. The situation impacted a recent update from the Muon g − 2 Theory Initiative, an effort by an international group of researchers to compile a standard-model prediction from the work of many teams [2]. In their update, the initiative decided to remove the discrepant data-driven values and to instead take an average solely from the lattice results (technically, this decision only applied to one part of the calculation called the hadronic vacuum polarization).
The initiative reported a muon anomalous moment of = 0.00116592033, with a precision of 540 parts per billion. This theoretical value agrees—within the error bars—with the final result from the Muon g − 2 Collaboration. “It seems likely that the g − 2 puzzle has been resolved,” says Thomas Blum from the University of Connecticut, a contributor to the Muon g− 2 Theory Initiative.
“This is a fantastic success of quantum field theory,” says Zoltán Fodor, a lattice researcher from Pennsylvania State University. The theory prediction involves summing the contributions from the three fundamental forces in the standard model: the electromagnetic, the weak, and the strong interactions. “It’s amazing that adding up very different types of calculations gives the same answer as the measurement up to 12 digits,” Fodor says.
Fodor admits that some people were hoping for a discrepancy, as that might have implied a new interaction or a new type of particle. But there have already been hints that such hopes might be disappointed. In 2021, the Budapest-Marseille-Wuppertal Collaboration, of which Fodor is the spokesperson, released a lattice calculation bringing theory and experiment closer together [3]. At the time, there were no other comparable results based on the lattice approach, so the theory community was hesitant. “What happened in the last year or so is that some independent groups have confirmed our results,” Fodor says.
But questions remain over the data-driven approach and its internal discrepancies. “More work on the data driven side is needed to understand the differences with experiments and with lattice QCD results,” Blum says. Winter agrees: “I would say it’s too early to claim that everything is resolved.”
The sure thing is that the Muon g − 2 experiment is finished. The large magnet is no longer being maintained at cryogenic temperatures, and there are no plans to repurpose the facility, Winter says. But muon physics continues. Fermilab is building a new experiment to look for rare muon-to-electron conversions, which are forbidden in the standard model. There is also a proposal to build a muon-magnetic-moment experiment at the Japan Proton Accelerator Research Complex using a different technique with a smaller magnet. “That would provide a strong cross-check of our experiment,” Winter says.
Wrapping up the Muon g − 2 experiment has been exciting but also a little sad, says Winter. “Many of us have worked over a decade on this experiment. It’s been a great experience and a great collaboration.”
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics Magazine based in Lyon, France.
References
- D. P. Aguillard et al., “Measurement of the positive muon anomalous magnetic moment to 127 ppb,” arXiv:2506.03069.
- R. Aliberti, “The anomalous magnetic moment of the muon in the Standard Model: An update,” arXiv:2505.21476.
- S. Borsanyi et al., “Leading hadronic contribution to the muon magnetic moment from lattice QCD,” Nature 593, 51 (2021).