Scientists at Fermilab, under the U.S. Department of Energy, have announced the final results of the long-running Muon g-2 experiment, achieving unprecedented precision in measuring the muon’s magnetic anomaly.
This conclusive result – delivered with a precision of 127 parts per billion – surpasses the project’s original design goals and confirms earlier findings released in 2021 and 2023.
The Muon g-2 experiment investigates a fundamental property of muons, subatomic particles similar to electrons but 200 times heavier. Like electrons, muons have a spin, behaving like tiny magnets that wobble in a magnetic field.
Measuring the rate of this wobble, or precession, provides a value known as the magnetic anomaly – critical for testing the limits of the Standard Model of particle physics.
Muon g-2: A legacy of precision and innovation
The origins of Muon g-2 trace back to earlier experiments at Brookhaven National Laboratory in the 1990s and early 2000s, which hinted at a potential discrepancy between experimental results and theoretical predictions.
This raised tantalising questions about the existence of unknown particles or forces – a window into “new physics” beyond the Standard Model.
To resolve these questions with greater accuracy, Brookhaven’s magnetic storage ring was transported across the country in 2013 to Fermilab in Illinois.
After a comprehensive upgrade, the experiment officially resumed in 2017, with the goal of dramatically improving precision.
Final result confirms but narrows the gap
Fermilab’s latest result, drawn from data collected between 2021 and 2023, consolidates previous datasets and benefits from critical refinements in experimental design.
The updated value of the muon’s magnetic anomaly is:
aμ = (g-2)/2 (muon, experiment) = 0.001 165 920 705 +- 0.000 000 000 114(stat.)
+- 0.000 000 000 091(syst.)
This measurement stands as the most precise determination of this quantity to date. The dataset was more than three times larger than that used in the 2023 release and incorporated enhanced beam quality and reduced uncertainties.
While the final value aligns with prior experimental results, it complicates the broader narrative. Initially, discrepancies between experimental and theoretical values hinted strongly at new physics.
However, recent theoretical recalculations using advanced computational methods – published by the Muon g-2 Theory Initiative – have moved closer to the experimental results, diminishing that gap.
The theory side: An evolving landscape
Theoretical physicists have been working in parallel to refine predictions of the muon’s magnetic anomaly.
In 2020, a major update based on data from other experiments deepened the discrepancy with Fermilab’s early measurements. But newer calculations, relying heavily on computational techniques, have since realigned theory closer to the observed data.
Despite this narrowing of the discrepancy, the situation remains unresolved. Two competing theoretical models now exist – one data-driven, one computational – with differing implications for new physics.
The Muon g-2 experiment now serves as a critical benchmark for evaluating these approaches, and future theoretical work will aim to reconcile them.
International collaboration drives success
The Muon g-2 collaboration, comprising 176 scientists from 34 institutions across seven countries, reflects an extraordinary convergence of expertise.
Unlike most high-energy physics experiments, Muon g-2 drew on a diverse array of specialists –not only particle physicists but also experts in accelerator, atomic, and nuclear physics.
This interdisciplinary teamwork was instrumental in achieving the experiment’s technical sophistication and ultimate success.
The inclusive and international character of the project demonstrates the global nature of modern scientific discovery and the importance of collaborative innovation in answering some of physics’ most fundamental questions.
What’s next for muon research?
Although Fermilab’s Muon g-2 experiment has concluded its primary analysis, its data legacy continues.
Upcoming research will explore other properties of the muon, such as its electric dipole moment and potential violations of charge, parity, and time-reversal symmetry – cornerstones of physical law.
In the early 2030s, a follow-up experiment at Japan’s Proton Accelerator Research Complex (J-PARC) aims to revisit the muon magnetic anomaly. However, it will initially operate at a lower precision than Fermilab’s benchmark-setting result.
Meanwhile, the Theory Initiative will persist in resolving discrepancies between competing predictions, ensuring that the precision achieved by Muon g-2 remains a central test of theoretical models for years to come.
Muon g-2: A benchmark for the future
The final result of the Muon g-2 experiment is more than just a number. It is the culmination of decades of scientific inquiry, a model of international collaboration, and a new standard in experimental precision.
While the narrowing gap between theory and experiment tempers early excitement about potential new physics, it reinforces the muon as a powerful probe into the fabric of reality and affirms the Standard Model’s resilience in the face of ever-tougher scrutiny.
The story of Muon g-2 doesn’t end here; it simply enters a new chapter in the quest to understand the Universe at its most fundamental level.
