Measurement of the Electron Magnetic Moment

The electron magnetic moment,$-\mu/\mu_B = g/2 = 1.001 \, 159 \, 652 \, 180 \, 59 \, (13) \, [0.13\, \rm{ppt}]$, is determined 2.2 times more accurately than the value that stood for 14 years. The most precisely determined property of an elementary particle tests the most precise prediction of the Standard Model (SM) to $1$ part in $10^{12}$. The test would improve an order of magnitude if the uncertainty from discrepant measurements of the fine structure constant $\alpha$ is eliminated since the SM prediction is a function of $\alpha$. The new measurement and SM theory together predict $\alpha^{-1}=137.035 \, 999 \, 166 \, (15) \, [0.11 \, \rm{ppb}]$ with an uncertainty ten times smaller than the current disagreement between measured $\alpha$ values.