Alberto Lusiani (INFN)  “Muon magnetic anomaly measurement to 0.46 ppm at FNAL”

The Dirac equation predicted that the g-factor of spin 1/2 particles be 2, but in 1948 Julian Schwinger calculated that quantum electrodynamics (QED) fluctuations increase the electron and muon g-factor such that the anomaly a_(μ) = (g_(μ) − 2)/2 = α/2π at first order. Since then, many theorists have estimated an increasingly larger amount of Standard Model contributions to a_(μ), and in 2020 an international collaboration of theoretical and experimental physicists (Muon g-2 Theory Initiative) has published a Standard-Model-based prediction of a_(μ) with a 0.37 ppm precision, using the available information and calculations. The theory prediction uncertainty is dominated by the hadronic contributions, which have been primarily estimated using measurements of hadronic processes with dispersive techniques.

The muon magnetic anomaly a_(μ) = (g_(μ) − 2)/2 has been measured since 1959 at CERN, and the current 0.54 ppm precision world average has been published in 2006, using data collected at BNL in the years 1997-2001.

While the electron magnetic anomaly is measured and predicted with about three orders of magnitude better precision, the muon magnetic moment test of the Standard Model is more sensitive to hadronic effects and to most hypothetical New Physics contributions because the muon mass is about 200 times larger than the electron one. The sensitivity of the muon anomaly measurement extends to many New Physics effects that are searched in high energy experiments.

After a 3-years long analysis of data collected in 2018 at FNAL, the Muon g-2 collaboration has completed a measurement of the muon magnetic anomaly with a slightly improved precision of 0.46 ppm. The updated world average has a precision of 0.35 ppm and differs from the theory prediction by 4.2 standard deviations. We describe how the measurement has been performed by measuring the muon precession frequency is a storage ring and the magnetic field experienced by the muons and how the new world average has been computed. The precession frequency measurement dominates the uncertainty and has been completed by 6 independent groups with 4 different methods, obtaining consistent results. The data analysis has been performed in a blind way with a
special care for the internal consistency and the estimation of systematic biases and uncertainties.

The Muon g-2 collaboration aims at reducing the statistical uncertainty by a factor 2 in about 18 months from now, using data already collected, and eventually aims at collecting a total of about 20 times the statistics of the BNL data, to reach an ultimate precision of 0.14 ppm (0.10 statistical and 0.10 systematic).
 
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