Researchers involved in the Muon g-2 experiment have unveiled their third determination of the muon’s magnetic anomaly. This latest outcome, boasting an enhanced precision of 127 parts-per-billion, aligns with their previously reported findings from 2021 and 2023, and notably surpasses the original experimental design target of 140 parts-per-billion.
A muon particle passing through lead in a cloud chamber. Image credit: Jino John 1996 / CC BY-SA 4.0.
The Muon g-2 investigation meticulously examines the gyroscopic motion of a fundamental subatomic particle known as the muon.
Muons bear a resemblance to electrons but are approximately 200 times more massive. Similar to electrons, muons possess a quantum mechanical characteristic termed spin, which can be conceptualized as a minuscule internal magnet.
When subjected to an external magnetic field, this intrinsic magnet exhibits a wobbling – or precessing – motion, akin to the axis of a spinning top.
The rate at which this precession occurs within a magnetic field is contingent upon specific properties of the muon, quantified by a value known as the g-factor.
Theoretical physicists derive the g-factor based on the current understanding of the universe’s fundamental workings, as encapsulated by the Standard Model of particle physics.
Approximately a century ago, the predicted value for g was 2. However, subsequent experimental measurements revealed a slight divergence from 2, resulting in a quantity termed the magnetic anomaly of the muon, denoted as aμ, which is calculated as (g-2)/2. The Muon g-2 experiment derives its nomenclature from this relationship.
The muon magnetic anomaly serves as a repository for the influences of all Standard Model particles, enabling theoretical physicists to compute these contributions with remarkable exactitude.
Nevertheless, prior measurements conducted at Brookhaven National Laboratory during the 1990s and 2000s indicated a potential discrepancy when compared to theoretical calculations of that era.
A divergence between experimental findings and theoretical predictions can serve as an indicator of novel physics.
Specifically, scientists speculated whether this disparity might be attributable to the influence of as-yet-undetected particles perturbing the muon’s precession.
Consequently, the decision was made to enhance the Muon g-2 experiment to achieve a measurement of superior precision.
In 2013, the magnetic storage ring from Brookhaven was relocated from Long Island, New York, to Fermilab in Batavia, Illinois.
Following years of substantial improvements and modifications, the Fermilab Muon g-2 experiment commenced operations on May 31, 2017.
Concurrently, an international consortium of theorists established the Muon g-2 Theory Initiative with the objective of refining the theoretical calculation.
In 2020, this Theory Initiative disseminated an updated, more precise Standard Model value derived from a methodology employing input data from complementary experiments.
The discrepancy associated with this particular technique persisted and widened in 2021 when Fermilab announced its inaugural experimental outcome, corroborating the Brookhaven result with a marginal enhancement in precision.
Simultaneously, a novel theoretical prediction emerged, based on an alternative methodology heavily reliant on computational resources.
This new theoretical figure was found to be closer to the experimental measurement, thereby reducing the observed discrepancy.
More recently, the Theory Initiative has published a revised prediction that synthesizes the findings of several research groups employing the aforementioned computational approach.
This updated prediction remains in closer proximity to the experimental measurement, diminishing the likelihood of new physics being implicated.
Nevertheless, the theoretical endeavors will persist in their pursuit of understanding the divergence between the data-driven and computational methodologies.
The most recent experimental determination of the muon’s magnetic moment from the Fermilab experiment is presented as follows:
aμ = (g-2)/2 (muon, experiment) = 0.001 165 920 705
This definitive measurement is predicated on the analysis of three years of collected data, spanning from 2021 to 2023, integrated with previously disseminated datasets.
This augmentation more than tripled the volume of data utilized for their second reported result in 2023, enabling the collaboration to finally achieve their precision objective established in 2012.
Furthermore, it represents an analysis derived from the experiment’s highest-quality data.
Near the conclusion of their second data acquisition period, the Muon g-2 Collaboration finalized refinements and upgrades to the experimental apparatus, which served to elevate the quality of the muon beam and mitigate uncertainties.
“The anomalous magnetic moment, or g-2, of the muon is significant as it offers a sensitive validation of the Standard Model of particle physics,” stated Regina Rameika, the Associate Director for the Office of High Energy Physics at the U.S. Department of Energy.
“This is an exciting outcome, and it is highly gratifying to witness an experiment reach a conclusive point with a precision measurement.”
“This long-anticipated result stands as a magnificent testament to precision and is poised to remain the world’s most accurate measurement of the muon magnetic anomaly for an extended duration.”
“Despite recent challenges encountered with theoretical predictions that diminish the evidence for new physics from muon g-2, this outcome provides a rigorous benchmark for proposed extensions to the Standard Model of particle physics.”
“This is an exceptionally thrilling juncture, as we have not only met but exceeded our objectives, a feat not easily accomplished in such precision measurements,” remarked Dr. Peter Winter, a physicist at Argonne National Laboratory and a co-spokesperson for the Muon g-2 Collaboration.
“With the steadfast support of funding agencies and our host facility, Fermilab, the undertaking has been overwhelmingly successful, achieving or surpassing virtually all of our targeted goals.”
“For over a century, g-2 has been instrumental in revealing profound truths about the nature of existence,” commented Professor Lawrence Gibbons of Cornell University.
“The addition of a precise measurement, which I believe will endure for a considerable time, is indeed exhilarating.”
“As it has been for decades, the magnetic moment of the muon continues to serve as a stringent standard by which the Standard Model is assessed,” stated Dr. Simon Corrodi, a physicist at Argonne National Laboratory.
“The novel experimental result illuminates this fundamental theory anew and will establish the benchmark for all subsequent theoretical calculations.”
The recently obtained results are slated for publication in the journal Physical Review Letters.
