Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm

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We 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 ω p ′ in a spherical water sample at 34.7 °C. The ratio ωa/ω p ′, 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.

Original languageEnglish
Article number141801
JournalPhysical Review Letters
Issue number14
StatePublished - Apr 7 2021

Bibliographical note

Funding Information:
We thank the Fermilab management and staff for their strong support of this experiment, as well as the tremendous support from our university and national laboratory engineers, technicians, and workshops. We are indebted to Akira Yamamoto, Lou Snydstrup, and Chien Pai who provided critical advice and engineering about the storage ring magnet and helped shepherd its transfer from Brookhaven to Fermilab. Greg Bock and Joe Lykken set the blinding clock and diligently monitored its stability. This result could not be interpreted without the worldwide theoretical effort to establish the standard model prediction, and in particular the recent work by the Muon Theory Initiative. The Muon Experiment was performed at the Fermi National Accelerator Laboratory, a U.S. Department of Energy, Office of Science, HEP User Facility. Fermilab is managed by Fermi Research Alliance, LLC (FRA), acting under Contract No. DE-AC02-07CH11359. Additional support for the experiment was provided by the Department of Energy offices of High Energy Physics and Nuclear Physics (USA), the National Science Foundation (USA), the Istituto Nazionale di Fisica Nucleare (Italy), the Science and Technology Facilities Council (UK), the Royal Society (UK), the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie Grant Agreements No. 690835 and No. 734303, the National Natural Science Foundation of China (Grants No. 11975153 and No. 12075151), MSIP, NRF, and IBS-R017-D1 (Republic of Korea), and the German Research Foundation (DFG) through the Cluster of Excellence (EXC 2118/1, Project ID 39083149).

Publisher Copyright:
© 2021 authors.

ASJC Scopus subject areas

  • Physics and Astronomy (all)


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