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Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm

Abstract

We present the first results of the Fermilab National Accelerator Laboratory (FNAL) Muon g − 2 Experiment for the positive muon magnetic anomaly a_μ ≡ (g_μ − 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 ω_a 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 ω_p' in a spherical wate sample at 34.7 °C. The ratio ω_a /ω_p', together with known fundamental constants determines a_μ(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 a_μ(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. DOI: 10.1103/PhysRevLett.126.141801

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