Researchers from the Muon G-2 Experiment have unveiled their third measurement of the Muon magnetic anomaly. The conclusive results align with findings published in 2021 and 2023 but boast significantly improved precision at 127 parts per billion, surpassing the experimental goal for 140 people.

The Muon G-2 experiment investigates the wobble of a fundamental particle known as the Muon.

Muons resemble electrons but are roughly 200 times more massive. Like electrons, they exhibit quantum mechanical properties called spins, which can be interpreted as tiny internal magnets.

When subjected to an external magnetic field, these internal magnets wobble akin to the axis of a spinning top.

The precession speed of a magnetic field is influenced by the muon’s characteristics, captured numerically as the G-factor.

Theoretical physicists derive G-factors based on our current understanding of the universe’s fundamental mechanics, as outlined in the standard model of particle physics.

Nearly a century ago, G was anticipated to be 2; however, experimental measurements revealed minor deviations from this value, quantified as the Muon magnetic anomaly, Aμ, based on the formula (G-2)/2, giving the Muon G-2 experiment its name.

Muon magnetic anomalies encapsulate the effects of all standard model particles, enabling theoretical physicists to compute these contributions with remarkable precision.

Earlier measurements conducted at the Brookhaven National Laboratory during the 1990s and 2000s indicated potential discrepancies with the theoretical calculations of that era.

Disparities between experimental results and theoretical predictions could signal the existence of new physics.

In particular, physicists contemplated whether these discrepancies could stem from an undetected particle influencing the muon’s precession.

Consequently, physicists opted to enhance the Muon G-2 experiments to obtain more accurate measurements.

In 2013, Brookhaven’s magnetic storage ring was relocated from Long Island, New York, to Fermilab in Batavia, Illinois.

Following extensive upgrades and enhancements, the Fermilab Muon G-2 experiment launched on May 31, 2017.

Simultaneously, an international collaboration among theorists established the Muon G-2 theory initiative aimed at refining theoretical calculations.

In 2020, the Theoretical Initiative released updated and more precise standard model values informed by data from other experiments.

The differences between the experimental results continued to widen in 2021 as Fermilab announced the initial experimental results, corroborating Brookhaven’s findings with improved accuracy.

Simultaneously, new theoretical predictions emerged, relying significantly on computational capabilities.

This information closely aligned with experimental measurements and narrowed the existing discrepancies.

Recently, the Theoretical Initiative published a new set of predictions integrating results from various groups using novel calculation techniques.

This result remains in close agreement with experimental findings and diminishes the likelihood of new physics.

Nevertheless, theoretical endeavors will persist in addressing the disparities between data-driven and computational approaches.

The latest experimental values for the muon magnetic moment from Fermilab’s experiments are:

a_μ =(g-2)/2 (Muon experiment) = 0.001 165 920 705

This final measurement is based on an analysis of data collected over the past three years, spanning 2021 to 2023, and is integrated with previously published datasets.

This has more than tripled the dataset size utilized in the second results from 2023, achieving the precision target set in 2012.

Moreover, it signifies the analysis of the highest quality data from the experiment.

As the second data collection run concluded, the Muon G-2 collaboration finalized adjustments and enhancements to the experiment, boosting muon beam quality and minimizing uncertainties.

“The extraordinary magnetic moment of the muon (G-2) is pivotal as it provides a sensitive test of the standard model of particle physics,” remarked Regina Lameika, associate director of high energy physics at the U.S. Department of Energy.

“This is an exhilarating result, and it’s fantastic to witness the experiment reach a definitive conclusion with precise measurements.”

“This highly anticipated outcome represents a remarkable achievement in accuracy and will hold the title of the most precise measurement of muon magnetic anomalies for the foreseeable future.”

“Despite recent theoretical challenges that have lessened the evidence for new physics in Muon G-2, this finding presents a robust benchmark for proposed extensions to the standard model of particle physics.”

“This is an incredibly exciting moment; not only did we meet our objectives, but we surpassed them, indicating that such precision measurements are challenging.”

“Thanks to Fermilab, the funding agencies, and the host lab, we accomplished our goals successfully.”

“For over a century, the G-2 has imparted crucial insights into the nature of reality,” stated Lawrence Gibbons, a professor at Cornell University.

“It’s thrilling to contribute accurate measurements that are likely to endure for a long time.”

“For decades, muon magnetic moments have served as a significant benchmark for the standard models,” noted Dr. Simon Kolody, a physicist at Argonne National Laboratory.

“The new experimental results illuminate this fundamental theory and establish a benchmark to guide new theoretical calculations.”

These new results will be featured in the journal Physical Review Letters.