Final report of the E821 muon anomalous magnetic moment measurement at BNL

TLDR

The muon anomalous magnetic moment receives contributions from QED, weak, and hadronic processes, with the largest theoretical uncertainty (~0.55 ppm) arising from hadronic vacuum polarization. The study reports the final precision measurement of the muon anomalous magnetic moment, a_μ. The measurement employed a storage ring at Brookhaven National Laboratory, using nearly equal samples of positive and negative muons, with detailed apparatus, data acquisition, and analysis procedures. The experiment determined a_μ(Expt)=11659208.0(5.4)(3.3)×10⁻¹⁰ with a combined 0.54 ppm uncertainty—a 14‑fold improvement over CERN—and the result lies 2.2–2.7 σ above standard‑model predictions.

Abstract

We present the final report from a series of precision measurements of the muon anomalous magnetic moment, ${a}_{\ensuremath{\mu}}=(g\ensuremath{-}2)/2$. The details of the experimental method, apparatus, data taking, and analysis are summarized. Data obtained at Brookhaven National Laboratory, using nearly equal samples of positive and negative muons, were used to deduce ${a}_{\ensuremath{\mu}}(\mathrm{\text{Expt}})=11659208.0(5.4)(3.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$, where the statistical and systematic uncertainties are given, respectively. The combined uncertainty of 0.54 ppm represents a 14-fold improvement compared to previous measurements at CERN. The standard model value for ${a}_{\ensuremath{\mu}}$ includes contributions from virtual QED, weak, and hadronic processes. While the QED processes account for most of the anomaly, the largest theoretical uncertainty, $\ensuremath{\approx}0.55\text{ }\text{ }\mathrm{ppm}$, is associated with first-order hadronic vacuum polarization. Present standard model evaluations, based on ${e}^{+}{e}^{\ensuremath{-}}$ hadronic cross sections, lie 2.2--2.7 standard deviations below the experimental result.