Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm
AbstractWe present the first results of the Fermilab National Accelerator Laboratory (FNAL)
Muon g − 2 Experiment for the positive muon magnetic anomaly
aμ ≡ (gμ − 2) /2.
The anomaly is determined from the precision measurements of two angular
frequencies. Intensity variation of high-energy positrons from muon decays directly
encodes the difference frequency ωa between the spin-precession and cyclotron
frequencies for polarized muons in a magnetic storage ring. The storage ring
magnetic field is measured using nuclear magnetic resonance probes calibrated in
terms of the equivalent proton spin precession frequency
in a spherical wate sample at 34.7 °C. The ratio
together with known fundamental constants determines
aμ(FNAL) = 116 592 040(54) × 10−11
(0.46 ppm). The result is 3.3
standard deviations greater than the standard model prediction and is in excellent
agreement with the previous Brookhaven National Laboratory (BNL) E821
measurement. After combination with previous measurements of both
μ+ and μ−,
the new experimental average of
aμ(Exp) = 116 592 061(41) × 10−11
(0.35 ppm) increases the tension between experiment and theory to 4.2 standard deviations.
Muon g-2 Collaboration,
Physical Review Letters, 126 (2021) 141801.
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