Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm (2104.03281v1)

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

• The paper reports a breakthrough measurement of the positive muon’s anomalous magnetic moment at 0.46 ppm precision using parity-violating decay observations.
• It employs rigorous magnetic field calibration with NMR probes to precisely determine muon spin-precession and cyclotron frequency differences.
• The results indicate a 3.3σ deviation from the Standard Model, further intensifying the search for new physics beyond established theories.

Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm

The research paper "Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm" presents a detailed experimental paper of the anomalous magnetic moment of the positive muon ($a_\mu$) conducted by the Fermilab Muon $g-2$ Experiment. This paper is crucial because the muon's magnetic moment, particularly its anomalous component, offers a window into the interactions that are both well established and those potentially beyond the Standard Model (SM).

Key Experimental Insights

The experimental setup leverages the well-established technique of observing parity-violating decays of polarized muons stored in a magnetic field. The anomalous precession frequency $\omega_a$ is determined through precision measurement of differences in angular frequencies, specifically the spin-precession frequency and the cyclotron frequency in a magnetic storage ring.

• Magnetic Field Calibration: The magnetic field, a fundamental factor for this measurement, is deduced through nuclear magnetic resonance (NMR) probes, calibrated in relation to the precession frequency of protons in a spherical water sample at a designated temperature.
• Precision and Corrections: The obtained ratio of frequencies, along with essential constants, yields the $a_\mu$ value. Notably, the measured $a_\mu = 116\,592\,040(54)\times 10^{-11}$ exhibits a precision of 0.46 parts per million (ppm).

Significant Numerical Results

A primary outcome of this experiment is that the measured value of $a_\mu$ surpasses the SM prediction by 3.3 standard deviations. This discrepancy persists even when considered with results from previous studies, such as the Brookhaven National Laboratory (BNL) E821 experiment. When these results are aggregated with other muon studies, the average deviates from the SM by 4.2 standard deviations, further accentuating the tension between experimental findings and theoretical expectations.

Implications and Future Directions

The outcome of this experiment has profound implications for theoretical physics, as any significant deviation such as the one observed can hint at physics beyond the SM, potentially pointing to new particles or forces. For practical implications, revising theoretical models could pave the way for new technologies and experimental methodologies in particle physics.

Additionally, this paper sets a precedential benchmark for precision in particle experiments. As technologies advance, similar methodologies could achieve even greater accuracy, refining our understanding of fundamental physics constants.

Theoretical Speculations

Looking ahead, these findings could catalyze the pursuit of theories extending beyond the SM, including supersymmetry or other new physics scenarios. With the increasing sophistication of Lattice Quantum Chromodynamics (QCD) and other simulation methods, there might be better alignment of theoretical predictions with experimental data in future endeavors.

In conclusion, this paper illustrates a significant stride in precision particle physics, elucidating important insights on muon behavior and its broader implications for theoretical physics, ultimately suggesting a departure from existing models and heralding new physics explorations.