February 8, 2000
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December 22, 2015
The magnetic moment is a fundamental property of particles. The measurement of these magnetic moments and the comparison with the values predicted by the standard model of particle physics is a way to test our understanding of the fundamental building blocks of our world. In some cases, such as for the electron, this comparison has resulted in confirmation of the standard model with incredible precision. In contrast, the magnetic moment of the muon has shown a long-standing d...
June 8, 2020
We review the present status of the Standard Model calculation of the anomalous magnetic moment of the muon. This is performed in a perturbative expansion in the fine-structure constant $\alpha$ and is broken down into pure QED, electroweak, and hadronic contributions. The pure QED contribution is by far the largest and has been evaluated up to and including $\mathcal{O}(\alpha^5)$ with negligible numerical uncertainty. The electroweak contribution is suppressed by $(m_\mu/M_...
February 19, 2009
The muon anomalous magnetic moment is one of the most precisely measured quantities in particle physics. In a recent experiment at Brookhaven it has been measured with a remarkable 14-fold improvement of the previous CERN experiment reaching a precision of 0.54ppm. Since the first results were published, a persisting "discrepancy" between theory and experiment of about 3 standard deviations is observed. It is the largest "established" deviation from the Standard Model seen in...
November 13, 2001
The status of the muon (g-2) experiment at the Brookhaven AGS is reviewed. An accuracy of 1.3 ppm on the mu^+ anomalous magnetic moment has been achieved and published. This result differs with the standard model prediction by about 2.5 standard deviations. A data sample with approximately seven times as much data is being analyzed, with a result expected in early 2001.
August 11, 2023
We present a new measurement of the positive muon magnetic anomaly, $a_\mu \equiv (g_\mu - 2)/2$, from the Fermilab Muon $g\!-\!2$ Experiment using data collected in 2019 and 2020. We have analyzed more than 4 times the number of positrons from muon decay than in our previous result from 2018 data. The systematic error is reduced by more than a factor of 2 due to better running conditions, a more stable beam, and improved knowledge of the magnetic field weighted by the muon d...
March 5, 2007
A review of the experimental and theoretical determinations of the anomalous magnetic moment of the muon is given. The anomaly is defined by a=(g-2)/2, where the Land\'e g-factor is the proportionality constant that relates the spin to the magnetic moment. For the muon, as well as for the electron and tauon, the anomaly a differs slightly from zero (of order 10^{-3}) because of radiative corrections. In the Standard Model, contributions to the anomaly come from virtual `loops...
August 12, 2002
Recent results announced as measurements of the muon's anomalous magnetic moment are in fact measurements of the muon's anomalous spin precession frequency. This precession frequency receives contributions from both the muon's anomalous magnetic and electric dipole moments. We note that all existing data cannot resolve this ambiguity, and the current deviation from standard model predictions may equally well be interpreted as evidence for new physics in the muon's anomalous m...
October 22, 2016
The precise measurement of the muon anomalous magnetic moment $a_\mathrm{\mu}$ has stimulated much theoretical and experimental efforts over more than six decades. The last experiment at Brookhaven National Laboratory, Upton, NY, USA obtained a value more than three standard deviations larger than predicted by the Standard Model of particle physics, and is one of the strongest hints for physics beyond the Standard Model. A new experiment at Fermi National Accelerator Laborato...
March 12, 2007
The muon anomalous magnetic moment is one of the most precisely measured quantities in particle physics. Recent high precision measurements (0.54ppm) at Brookhaven reveal a ``discrepancy'' by 3 standard deviations from the electroweak Standard Model which could be a hint for an unknown contribution from physics beyond the Standard Model. This triggered numerous speculations about the possible origin of the ``missing piece''. The remarkable 14-fold improvement of the previous ...
September 2, 2003
The muon g-2 collaboration has measured the anomalous magnetic g value of the positive muon to within a relative uncertainty of 0.7 parts per million. The result, a_{\mu^+} = 11 659 204(7)(5) x 10^{-10} is in good agreement with the preceding data on a_{\mu^+} and a_{\mu^-} and has about twice smaller uncertainty. The measurement tests standard model theory, which at the level of the experimental uncertainty involves quantum electrodynamics, quantum chromodynamics, and electr...