The Fermilab Muon g-2 collaboration released its final measurement in June 2025, delivering the most precise determination of the muon's magnetic anomaly to date. The result, based on the experiment's full Run-2 and Run-3 dataset, achieves a precision of 127 parts per billion — surpassing the original design goal of 140 ppb — and agrees with the earlier Brookhaven and Fermilab measurements. The experimental world average now stands at aμ = 0.001 165 920 705 ± 0.000 000 000 114 (stat) ± 0.000 000 000 091 (syst).
That precision sharpens the long-standing discrepancy with the data-driven Standard Model prediction, which relies on electron-positron collider measurements of the hadronic vacuum polarization. The gap between experiment and the dispersive (R-ratio) theory value now reaches 5.1 standard deviations, the strongest tension yet in a puzzle that has persisted since the Brookhaven result in 2001.
What's New
The June 2025 paper, submitted to Physical Review Letters, more than triples the analyzed dataset compared to the 2023 result. Improvements to the muon beam and systematic controls in the final running period enabled the collaboration to reach its target precision. The measurement will remain the world's most precise for years; a follow-up experiment at Japan's J-PARC facility is not expected to match this precision until the early 2030s.
Meanwhile, the theoretical landscape has shifted. The Muon g-2 Theory Initiative's latest combined lattice QCD prediction — a computational approach that simulates the strong force on supercomputers — now aligns closely with the experimental value, suggesting no new physics is required. But that agreement comes at a cost: the lattice result disagrees with the data-driven dispersive method at a level that cannot be explained by known uncertainties. Recent measurements from the VEPP-2000 collider in Novosibirsk, published in 2023, produced a pion production rate that aligns with the lattice calculation, deepening the split between the two theoretical approaches.
Why It Matters
The muon's magnetic anomaly has served as one of the most sensitive probes of the Standard Model for decades. A confirmed discrepancy would point to undiscovered particles or forces — candidates for dark matter, explanations for the matter-antimatter asymmetry, or other physics beyond the current framework. The Fermilab result cements the experimental side with unprecedented rigor. The burden now falls on theorists to resolve why two independent methods for calculating the same Standard Model contribution — lattice QCD and the dispersive R-ratio approach — yield mutually incompatible answers. Until that internal theoretical conflict is settled, the question of whether the muon's wobble signals new physics remains open.
Our Take
The experimental program has delivered on its promise: a sub-130 ppb measurement that will stand as the benchmark for a generation. The real crisis has migrated from experiment-theory tension to theory-theory tension. If the lattice QCD consensus holds, the anomaly disappears — but only if the dispersive community can identify a systematic error in decades of collider data. That is a harder problem than refining a single experiment, and it may take years of cross-checks between lattice groups and new R-ratio measurements to resolve.
