In June 2025, the Muon g–2 collaboration at Fermilab reported its third and last estimation of the muon odd attractive minute, based on information collected from 2020 to 2023.
News
Here are the key points:
Precision Achieved
The last estimation come to a accuracy of 127 parts per billion (ppb).
Johannes Gutenberg College Mainz
+2
GlobeNewswire
+2
This outperforms the unique plan objective of the explore, which was 140 ppb.
Johannes Gutenberg College Mainz
The add up to instability (combining measurable, orderly, and outside) gives
𝑎
𝜇
=
0.001165920705
±
(
1.14
×
10
−
10
)
𝑠
𝑡
𝑎
𝑡
±
(
0.91
×
10
−
10
)
𝑠
𝑦
𝑠
𝑡
±
(
0.26
×
10
−
10
)
𝑒
𝑥
𝑡
a
μ
=0.001165920705±(1.14×10
−10
)
stat
±(0.91×10
−10
)
syst
±(0.26×10
−10
)
ext
which compares to the cited 127 ppb.
IDW Nachrichten
In outright terms, based on their combined data:
𝑎
𝜇
=
1165920715
(
145
)
×
10
−
12
a
μ
=1165920715(145)×10
−12
(where “145” is the instability in units of 10⁻¹²) for the world normal, overwhelmed by Fermilab’s information.
muon-g-2.fnal.gov
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Data Sample
The modern result incorporates more than 2.5 times the measurements of their past distributed dataset.
muon-g-2.fnal.gov
+1
Over the full run (2018–2023), they collected 308 billion muons.
Johannes Gutenberg College Mainz
+1
The estimation utilized their highest-quality information from the last a long time, taking advantage of changes in the muon pillar, superior calibration, and refined systematics.
The Office of Energy's Energy.gov
Agreement with Past Results
The unused result adjusts exceptionally well with prior Fermilab comes about from 2021 and 2023, asserting consistency.
Johannes Gutenberg College Mainz
Because of this consistency, the last result reinforces the world‑average esteem of
𝑎
𝜇
a
μ
.
INFN
+1
The collaboration submitted its result to Physical Audit Letters.
News
Experimental Achievement
Reaching 127 ppb is broadly respected as a major victory in accuracy estimation. As the DOE’s Argonne lab put it: “an test come to a conclusive conclusion with a exactness measurement.”
Argonne National Laboratory
The numerous groups in the collaboration (quickening agent physicists, locator groups, reenactment / investigation) all worked to diminish vulnerabilities, cross-check impacts, and cleanly extricate the muon precession recurrence.
Johannes Gutenberg College Mainz
Why This Still Challenges Theorists
Despite the exploratory triumph, scholars aren’t totally at ease: the result still raises vital hypothetical issues. Here are the fundamental reasons.
Theoretical Forecast Uncertainty
Theoretical forecasts for
𝑎
𝜇
a
μ
come from the Muon g‑2 Hypothesis Activity (and other bunches), which combine data-driven strategies (from e⁺e⁻ collision information) and cross section QCD (computational) approaches.
Phys.org
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A modern hypothetical expectation, distributed as of late, employments an upgraded computational (cross section) procedure. That unused hypothesis esteem is closer to the Fermilab test result, decreasing the already claimed error.
Argonne National Laboratory
+1
However, the instability in the hypothetical forecast is still approximately four times bigger than the exploratory instability.
Phys.org
Because of this, indeed in spite of the fact that the numbers presently cover (i.e., the hypothesis and try are steady inside mistake bars), it’s not authoritatively settled whether there is any genuine “anomaly” or not. Aida X. El‑Khadra (chair of the Hypothesis Activity) has emphasized that the address “Does the SM concur or disagree?” is still not settled to everyone’s fulfillment.
Phys.org
Hadronic Vacuum Polarization (HVP)
One of the most fragile commitments to
𝑎
𝜇
a
μ
comes from hadronic vacuum polarization: quantum changes including quarks and gluons (solid drive) contribute by means of circle charts. These are famously troublesome to compute.
Phys.org
Discrepancies stay between diverse hypothetical approaches:
Data-driven (dispersive) strategies utilize test cross-section data (e.g., e⁺e⁻ → hadrons) to gather HVP contributions.
Lattice QCD does a first-principles computation on a space-time lattice.
The unused lattice-based forecast is closer to the test result, but that doesn’t completely dispose of pressure, since orderly instabilities and methodological issues in HVP stay dynamic regions of inquire about.
Phys.org
Theorists are proceeding to refine both methods.
Room for Modern Material science Decreased (but Not Closed)
Earlier, the muon g–2 inconsistency was considered a solid indicate for unused material science (supersymmetry, additional Higgs bosons, dark-matter–sector particles, etc.).
Phys.org
With the modern hypothesis forecasts moving closer to the exploratory estimation, the case for “beyond the Standard Model” (BSM) clarifications is debilitated, at slightest in the setting of muon g–2.
GlobeNewswire
+1
But it’s not ruled out: any new-physics demonstrate presently has to fight with a exceptionally tight test benchmark. The progressed accuracy of the estimation implies that unused hypotheses must create adjustments inside this limit window — a much more rigid limitation than before.
Long-Term Hypothetical Exertion Required
According to specialists, scholars will require to proceed lessening the blunder bars. The test result sets a benchmark that future hypothetical forecasts must coordinate or clarify.
The Office of Energy's Energy.gov
The Muon g-2 Hypothesis Activity and related endeavors are likely to keep working on cross section QCD, refined dispersive strategies, and cross-checks between approaches.
Phys.org
There is moreover intrigued in investigating indeed heavier leptons, like the tau. But measuring the tau’s atypical attractive minute is massively challenging: the tau’s lifetime is greatly brief, making accuracy tests exceptionally troublesome.
Phys.org
Future Exploratory Prospects
While this Fermilab result will stay the world’s most exact estimation for numerous a long time, there are plans for future tests. For occasion, at the Japan Proton Quickening agent Investigate Complex (J-PARC), a muon g–2 estimation is expected in the early 2030s.
The Office of Energy's Energy.gov
However, the anticipated accuracy of early J-PARC runs may not promptly coordinate Fermilab’s 127 ppb.
The Division of Energy's Energy.gov
Meanwhile, any new-physics hypothesis must presently survive this high-precision “bar” but may too be tried advance as hypothetical strategies move forward or unused tests come online.
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