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[over ˜]_{p}^{'} in a spherical water sample at 34.7 °C. The ratio ω_{a}/ω[over ˜]_{p}^{'}, together with known fundamental constants, determines a_{μ}(FNAL)=116 592 040(54)×10^{-11} (0.46ppm). 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} (0.35ppm) increases the tension between experiment and theory to 4.2 standard deviations.
Highlights
The magnetic moments of the electron and muon μ⃗ l 1⁄4 gl q 2ml s⃗where gl 1⁄4 2ð1 þ alÞ (l 1⁄4 e, μ) have played an important role in the development of the standard model (SM)
Motivated in part by anomalies in the hyperfine structure of hydrogen [2,3], Schwinger [4] proposed an additional contribution to the electron magnetic moment from a radiative correction, predicting the anomaly [5] ae 1⁄4 α=2π ≃ 0.001 16 in agreement with experiment [6]
While the electron magnetic anomaly has been measured to fractions of a part per billion [57], the relative contribution of virtual heavy particles in many cases scales as ðmμ=meÞ2 ≃ 43 000
Summary
Where gl 1⁄4 2ð1 þ alÞ (l 1⁄4 e, μ) have played an important role in the development of the standard model (SM). A more precise experiment [9] confirmed Schwinger’s prediction for the muon anomaly and thereby established for the first time the notion that a muon behaved like a heavy electron in a magnetic field. This evidence, combined with the discovery of the muon neutrino [10], pointed to the generational structure of the SM. While the electron magnetic anomaly has been measured to fractions of a part per billion [57], the relative contribution of virtual heavy particles in many cases scales as ðmμ=meÞ2 ≃ 43 000 This is the case e.g. for the W and Z bosons of the SM and many hypothetical new particles, and it gives the muon anomaly a significant advantage when searching for effects of new heavy physics. We report our first result with a precision of 0.46 ppm
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