What it is
The Muon g-2 Experiment at Fermi National Accelerator Laboratory presents a new measurement of the magnetic anomaly of the positive muon, a_μ, from data taken from 2020 to 2023, a dataset with over 2.5 times the total statistics of its previous results. From the ratio of the precession frequencies of muons and protons in its storage-ring magnetic field, together with precisely known ratios of fundamental constants, the collaboration determines a_μ = 1 165 920 710(162) × 10⁻¹² (139 ppb) for the new datasets, and 1 165 920 705(148) × 10⁻¹², a precision of 127 ppb, when combined with its previous results. The new experimental world average, dominated by the measurements at Fermilab, is 1 165 920 715(145) × 10⁻¹² (124 ppb), and the Fermilab measurements have improved the precision of the world average by over a factor of 4.
Why it matters
Fermilab's final result measures the muon's magnetic anomaly to 127 ppb; its text says the world average agrees with the Theory Initiative's 2025 Standard Model value. The paper describes the anomaly as a precision probe of the Standard Model, sensitive to particles and interactions within it and potentially beyond it, and an earlier Brookhaven measurement had shown a statistically significant tension with the predictions of its time. Drawn from the experiment's full six years of data, this is, in the paper's words, the most precise determination of the anomaly and a powerful benchmark for extensions of the Standard Model.
Underlined numbers link to their source. Every metric and quoted figure is listed under Sources and data below.
Filed underparticle physics, muon g-2, Standard Model, precision measurement, Fermilab
