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    Muon g-2 experiment

    Also known as E989, Fermilab Muon g-2, muon anomalous magnetic moment

    Muon g-2 was a Fermilab experiment that measured the magnetic anomaly of the muon, a heavier cousin of the electron; its final June 2025 result reached a precision of 127 parts per billion.[1][2] A 2025 Standard Model prediction based on lattice QCD agrees with the measurement, largely closing a long-running hint of new physics, though theorists' two methods still disagree.[3][4]

    Editor reviewedUpdated Particle physics and cosmologyPhysicsScience
    Key facts

    What it measured

    Muons are particles similar to electrons but about 200 times more massive.[1] The experiment measured the muon’s magnetic anomaly, a small quantity that the Standard Model predicts precisely, so any gap between measurement and prediction could point to unknown particles or forces.[2][3]

    A muon’s spin makes it behave like a tiny magnet. In a magnetic field that magnet wobbles, or precesses, like the axis of a spinning top, at a rate set by a number called the g-factor. Nearly a century ago g was predicted to be exactly 2, but measurements showed it is slightly larger. The difference, (g-2)/2, is the magnetic anomaly that gives the experiment its name.[5] The anomaly encodes the effects of every particle in the Standard Model, which makes it a test of the model as a whole.[5] Direct searches for new particles happen instead at high-energy colliders such as the large-hadron-collider.[6]

    History

    Measurements at Brookhaven National Laboratory in the late 1990s and early 2000s hinted at a discrepancy with the theory of the time.[7] The experiment’s storage ring was then moved from Brookhaven in New York to Fermilab in Illinois in 2013, and after upgrades the Fermilab experiment started up on 31 May 2017.[8][7] Its first result, in 2021, confirmed Brookhaven’s value.[7] The collaboration has nearly 176 scientists from 34 institutions in seven countries, and its final result used data from 2021 to 2023.[9] Unusually for particle physics, the team also included accelerator, atomic and nuclear physicists.[10]

    The final result

    On 3 June 2025 the collaboration released its final measurement. It reached a precision of 127 parts per billion, better than the 140 ppb design goal.[2] The value of the muon’s magnetic anomaly is 0.001 165 920 705, with statistical and systematic uncertainties of 114 and 91 in the last digits.[11] The final dataset was more than three times the size of the one used for the 2023 result. Fermilab expects the measurement to stay the world’s most precise for many years and to serve as a strict benchmark for any proposed extension of the Standard Model.[12]

    Theory catches up

    For years the measurement appeared to disagree with the Standard Model prediction. A second theoretical technique, based on heavy computer simulation, began narrowing the gap in 2021.[7] In May 2025 the Muon g-2 Theory Initiative published an updated prediction, and the difference from experiment is now 38 ± 63 × 10⁻¹¹, which is no significant tension.[3] The change came because new CMD-3 electron-positron data made data-driven evaluations of the hadronic vacuum polarisation impossible to reconcile. The initiative therefore adopted lattice-QCD calculations for that term.[13]

    The question is not fully closed. The 2020 data-driven prediction and the 2025 lattice prediction disagree with each other by about three standard deviations.[4] An experiment at J-PARC in Japan is expected to measure the anomaly in the early 2030s, initially with lower precision than Fermilab’s.[14]

    Questions readers ask

    Did Muon g-2 find new physics?

    Probably not. The 2025 Standard Model prediction differs from the experimental average by 38 ± 63 × 10⁻¹¹, which is no significant tension.[3]

    Why did the theory prediction change?

    New CMD-3 measurements made the older data-driven method impossible to reconcile, so the 2025 update switched to lattice-QCD calculations for the key hadronic contribution.[13]

    Is the question closed?

    Not fully. The 2020 data-driven and 2025 lattice predictions disagree by about three standard deviations, and an experiment at J-PARC in Japan plans another measurement in the early 2030s.[4][14]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      Muons are particles similar to electrons but about 200 times more massive. confirmedas of 2025-06-03

    2. [2]

      On 3 June 2025 Fermilab's Muon g-2 experiment released its final measurement of the muon's magnetic anomaly, with a precision of 127 parts per billion, better than its 140 ppb design goal. confirmedas of 2025-06-03

    3. [3]

      The Muon g-2 Theory Initiative's 2025 Standard Model prediction differs from the experimental average by 38 plus or minus 63 (×10⁻¹¹), showing no significant tension between theory and experiment. confirmedas of 2025-05-27

    4. [4]

      The Theory Initiative's 2020 data-driven prediction and its 2025 lattice-QCD-based prediction disagree at the level of about three standard deviations, leaving open whether a sign of new physics exists. confirmedas of 2025-07-08

    5. [5]

      Muon g-2 measures how a muon's spin, which acts like a tiny magnet, wobbles (precesses) in a magnetic field; the rate depends on the g-factor, which was predicted to be 2 nearly 100 years ago but is slightly larger, and the deviation, (g-2)/2, is the muon magnetic anomaly, which encodes the effects of all Standard Model particles. confirmedas of 2025-06-03

    6. [6]

      The LHC collides protons at 13.6 TeV, the highest energy reached in a laboratory. confirmedas of 2026-10-10

    7. [7]

      Brookhaven measurements in the late 1990s and early 2000s hinted at a discrepancy with theory; after the ring moved to Fermilab, the experiment started up on 31 May 2017, and its 2021 first result confirmed Brookhaven's while a new lattice-based prediction narrowed the gap. confirmedas of 2025-06-03

    8. [8]

      The Muon g-2 storage ring was moved from Brookhaven National Laboratory in New York to Fermilab in 2013, and data-taking began in 2017. confirmedas of 2025-07-08

    9. [9]

      The Muon g-2 collaboration has nearly 176 scientists from 34 institutions in seven countries; its final result used data taken from 2021 to 2023. confirmedas of 2025-06-03

    10. [10]

      Unusually for a particle physics experiment, the Muon g-2 collaboration included accelerator, atomic and nuclear physicists as well as high-energy physicists. confirmedas of 2025-06-03

    11. [11]

      The final Fermilab value of the muon magnetic anomaly is 0.001 165 920 705, with a statistical uncertainty of 0.000 000 000 114 and a systematic uncertainty of 0.000 000 000 091. confirmedas of 2025-06-03

    12. [12]

      Fermilab expects the final Muon g-2 result to remain the world's most precise measurement of the muon magnetic anomaly for many years and a stringent benchmark for extensions of the Standard Model; the final dataset more than tripled the data used for the 2023 result. confirmedas of 2025-06-03

    13. [13]

      The 2025 theory update switched to lattice-QCD calculations for the leading hadronic contribution, because new CMD-3 measurements made data-driven evaluations impossible to reconcile. confirmedas of 2025-05-27

    14. [14]

      A muon g-2 experiment at J-PARC in Japan is expected to make another measurement in the early 2030s, initially with lower precision than Fermilab's. confirmedas of 2025-06-03

    Revision history (2)
    1. Page created.
    2. Explained the spin-precession method; added Brookhaven history, the 2017 start, the 2021 first result and Fermilab's benchmark statement.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "Muon g-2 experiment." ContentLora, updated Oct 10, 2026. https://contentlora.com/wiki/muon-g-2-experiment

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