In April 2026 the Breakthrough Prize in Fundamental Physics, worth $3 million, went to three generations of the Muon g-2 experiment: hundreds of scientists at CERN, Brookhaven and Fermilab who spent over sixty years measuring one number about one particle[1][2]. Why would that be worth a prize sometimes called the “Oscars of Science”? Because that one number is a test of almost everything we know about particle physics.
The particle: a heavy electron
A-Level extensionThe muon is a lepton, in the same family as the electron in the Standard Model you study at A-Level. It has the same charge (−1) but about 207 times the mass, and it is unstable, decaying in about 2.2 microseconds when at rest. Like the electron it has spin, so it behaves like a tiny magnet. Put it in a magnetic field and its spin axis rotates, or precesses, like a wobbling spinning top.
Why “g minus 2”?
The number g describes how strong that tiny magnet is for a given spin. Paul Dirac's theory predicted g = 2 exactly. But quantum physics says empty space is full of short-lived virtual particles popping in and out of existence, and the muon interacts with all of them. Each one nudges g slightly above 2. The anomaly, a = (g − 2)/2, is the sum of every such contribution: electromagnetic, weak and strong forces, and any undiscovered particles too. Measure it precisely, calculate it precisely, and compare. A mismatch would hint at new physics.
Muons circle a storage ring in a uniform magnetic field. If g were exactly 2, the spin (red) would stay locked to the direction of travel (grey). Because g is slightly above 2, the spin gets a little further ahead on every lap (hugely exaggerated here). Measuring that drift measures g − 2.
How the experiment works
The clever trick is to measure the difference directly. Muons are stored in a 50-foot (about 15 m) superconducting ring. The ring was built for the Brookhaven experiment and moved to Fermilab in Illinois in 2013 on a 35-day, 3,200-mile journey by land and sea[2]. Inside the ring's uniform magnetic field, the spin rotates slightly faster than the muon goes round. When each muon decays, its electron or positron tends to fly out in the direction of the spin, so detectors around the ring can track how far the spin has drifted. The drift rate depends on (g − 2) and on the magnetic field, which is measured with extreme care.
Timeline
- 1959First muon g-2 experiments begin at CERN.
- Mid-1990sBrookhaven National Laboratory runs the second generation.
- 2013The storage ring is shipped 3,200 miles to Fermilab.
- 2018–2023Fermilab collects data in six runs.
- June 2025Final result: 127 parts per billion precision.
- April 2026Breakthrough Prize in Fundamental Physics.
The final number
On 3 June 2025 the Fermilab collaboration, nearly 176 scientists from 34 institutions in seven countries, announced[3]:
aμ = 0.001 165 920 705 ± 0.000 000 000 114 (stat.) ± 0.000 000 000 091 (syst.)
That is a precision of 127 parts per billion, beating the design goal of 140 ppb[3]. Notice how the uncertainty is reported in two parts: statistical (from the finite number of muons) and systematic (from imperfections in the apparatus, such as knowing the field exactly). That is exactly the distinction your practical-skills endorsement asks you to make.
Quick check
127 parts per billion is the same as which percentage uncertainty?
Anomaly solved, or not?
The distinction students often miss
A disagreement can be resolved by better theory, not only by new particles.
Exam-style question
The muon has a mean lifetime of 2.2 μs at rest. In the g-2 storage ring muons travel at about 0.9994c and their measured lifetime is about 64 μs. Explain this observation. [3 marks]
Show model answer and mark breakdown
- the muons are moving at close to the speed of light, so special relativity applies (time dilation) (1 mark)
- the muon's lifetime measured in the lab frame is longer than its proper lifetime: t = γt₀ (1 mark)
- γ = 1/√(1 − 0.9994²) ≈ 29, so t ≈ 29 × 2.2 μs ≈ 64 μs (1 mark)
Examiner focus: name time dilation, use the correct equation, and show the calculation of the Lorentz factor.
Common mistake: saying the muons 'age more slowly because they are cold' or using length contraction without explaining which frame is which.
The storage ring runs muons at a special “magic” momentum, which is why the time dilation works out so neatly. Relativity is not just for exams: our post on why GPS needs Einstein shows it in your pocket.
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Sources and further reading
- University of Washington: UW physicists win 2026 Breakthrough Prize for study of enigmatic particle (21 Apr 2026) — prize, three generations, CERN 1959
- Fermilab: Experiment receives Breakthrough Prize in Fundamental Physics (Apr 2026) — the storage ring's journey
- Fermilab: Muon g-2 announces most precise measurement of the magnetic anomaly of the muon (Jun 2025) — final value, 127 ppb, collaboration size, theory comparison
- Physics World: Muon g-2 achieves record precision, but theoretical tensions remain — theory context
Last reviewed: 5 October 2026. Figures are typical published values and rules can change. Exam wording varies by specification, so check yours.
Frequently asked questions
What is a muon?
A fundamental particle, like a heavier cousin of the electron: same charge, about 207 times the mass, and unstable, decaying in about 2.2 microseconds when at rest. Muons are made naturally when cosmic rays hit the atmosphere, and in accelerators.
What does 'g-2' mean?
g measures how strongly a particle's spin behaves like a magnet. Simple quantum theory predicts g = 2 exactly. In reality, the particle constantly interacts with short-lived 'virtual' particles, which nudges g slightly above 2. The size of that nudge, the anomaly a = (g − 2)/2, tests whether we know about every particle that can contribute.
What was the final result?
In June 2025 Fermilab's Muon g-2 experiment announced a = 0.001 165 920 705 with a precision of 127 parts per billion, better than its 140 ppb design goal. It agreed with the experiment's previous results.
Did it find new physics?
Probably not. For years the measurement disagreed with the main theoretical prediction, hinting at unknown particles. But improved theory using lattice QCD on supercomputers now agrees closely with the measurement, so the 'anomaly' has largely gone away, though different theory methods are still being reconciled.
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Sudershan Soni
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