Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm

TL;DR

This paper reports a highly precise measurement of the positive muon anomalous magnetic moment, revealing a significant deviation from the Standard Model prediction, which could indicate new physics beyond current theories.

Contribution

First measurement of the muon g-2 at Fermilab with 0.46 ppm precision, confirming previous results and increasing the discrepancy with the Standard Model.

Findings

Measured $a_$ with 0.46 ppm accuracy.

Result exceeds Standard Model prediction by 3.3 standard deviations.

Combined data increases the tension to 4.2 standard deviations.

Abstract

We present the first results of the Fermilab Muon g-2 Experiment for the positive muon magnetic anomaly $a_\mu \equiv (g_\mu-2)/2$. 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 $\omega_a$ 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 ${\tilde{\omega}'^{}_p}$ in a spherical water sample at 34.7$^{\circ}$C. The ratio $\omega_a / {\tilde{\omega}'^{}_p}$, together with known fundamental constants, determines $a_\mu({\rm FNAL}) = 116\,592\,040(54)\times 10^{-11}$ (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 $\mu^+$ and $\mu^-$, the new experimental average of $a_\mu({\rm Exp}) = 116\,592\,061(41)\times 10^{-11}$ (0.35\,ppm) increases the tension between experiment and theory to 4.2 standard deviations