What it is
The Muon g-2 collaboration at Fermilab released its third and final measurement of the positive muon's magnetic anomaly, drawing on data taken from 2020 to 2023 with more than 2.5 times the statistics of its previous results. Combining all of its data, the experiment reports a_mu = 116,592,070.5 x 10^-12, a precision of 127 parts per billion. Folded into the global picture, this yields an experimental world average of 116,592,071.5 x 10^-12 at 124 ppb, with the Fermilab program improving the world-average precision by more than a factor of four.
Why it matters
The muon's magnetic anomaly is among the most precisely measured quantities in all of physics, and for years its experimental value appeared to diverge from Standard Model predictions, hinting at undiscovered particles. This final result pins the number down to 127 ppb, and as lattice-QCD theory converges toward it, the anomaly that once looked like a crack in the Standard Model is closing. The factor of four gain over prior world averages makes this the anchor value for a decade-long stress test of fundamental physics.
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Filed underparticle physics, muon g-2, Standard Model, precision measurement, Fermilab