The Muon g-2 experiment has revealed its third, conclusive, and most accurate measurement of the muon’s magnetic anomaly, a value that illuminates fundamental forces and particles operating within the cosmos.
An international group of researchers, including physicists from the University of Michigan, has achieved the most accurate determination to date of a crucial, oscillating characteristic of a subatomic particle known as the muon.
Engaged in the Muon g-2 project, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, the team has published its third and final assessment of the muon’s magnetic anomaly.
“This is a critical area to explore for nature to unveil secrets we haven’t yet discovered,” stated Timothy Chupp, a physics professor leading the U-M team’s efforts in the Muon g-2 experiment. “Evaluating this magnetic characteristic with such precision and juxtaposing it against theoretical frameworks is how we can investigate new types of interactions that influence the evolution of the universe and its constituents.”
The muon magnetic anomaly, commonly referred to as g-2 (pronounced “gee minus two”), is the namesake of the experiment. This value derives from measuring the oscillation of the muon’s magnetic pole alongside the intensity of the magnetic field. Mathematically, the magnetic anomaly is symbolized as am and is equal to g-2 divided by 2.
With this recent and conclusive measurement, the collaboration builds upon the accuracy it reported for the g-2 value in 2021 and 2023. The new, final outcome aligns with those earlier findings, but with significantly improved precision that even exceeds the original design objectives of the experiment.
Precision is akin to golf in that a lower score denotes superior performance. In its 2023 release, the Muon g-2 team announced a precision of 200 parts per billion. With their latest findings, this has enhanced to 127 parts per billion, surpassing the design goal of 140 parts per billion.
“This is a very thrilling moment because we not only met our aims but exceeded them, which is not an easy feat for these precision measurements,” remarked Peter Winter, a physicist at Argonne National Laboratory and co-spokesperson for the Muon g-2 collaboration. “With the backing of the funding agencies and the host lab, Fermilab, this has been highly successful overall, as we achieved or exceeded nearly all our targets.”
The measured value of g-2 may or may not suggest new physics, such as yet-to-be-discovered particles. That determination will rely on scientists who are working to compute what the theoretical value of g-2 should be. Currently, there are two methods that provide differing results.
The second, more recent value does shift the expectation away from the necessity for new physics to clarify the results. As theorists continue their work, however, the Muon g-2 Experiment has accomplished its task of offering a sufficiently precise experimental value for comparison.
“It’s an exhilarating result and gratifying to see an experiment conclude definitively with a precise outcome,” said Regina Rameika, the U.S. Department of Energy’s Associate Director for the Office of High Energy Physics.
The Muon g-2 collaboration announced the outcomes on June 3 and has submitted the report to the journal Physical Review Letters.
Michigan’s magnetic moment
The Muon g-2 collaboration consists of nearly 176 scientists from 34 institutions across seven countries. The extensive reach of the experiment was complemented by the diverse technical expertise of its members. This is not always the case for large experiments, but it played a crucial role in the success of Muon g-2, according to co-spokesperson Marco Incagli, a physicist at the Italian National Institute for Nuclear Physics in Pisa.
“This experiment is quite unique because it incorporates a variety of elements,” Incagli noted. “It is genuinely a collaborative effort among communities that typically engage in different experiments.”
David Aguillard, a graduate student collaborating with Chupp at U-M, concurred.
“Determining a fundamental characteristic of the muon with the precision and accuracy achieved by the g-2 collaboration is a monumental undertaking that draws on diverse areas of expertise,” Aguillard stated. His work was instrumental in validating the muon g-2’s magnetic field measurement, one of two essential experimental values required to ascertain g-2.
“My work comprises one of many cross-checks that extend beyond the measurement itself, which are crucial for ensuring the accuracy of the final outcome,” Aguillard added.
Another segment where the U-M team made contributions was led by Eva Kraegeloh, a graduate student pursuing a dual doctorate in physics and scientific computing. She spearheaded the analysis that merged maps of the magnetic field with the muons’ locations within the ring, enabling researchers to comprehend how the muons interacted with the magnetic field in Muon g-2’s 50-foot ring.
“Working alongside so many talented individuals toward a shared objective has been incredibly rewarding, not only in advancing science but also on a personal level,” Kraegeloh expressed. “I’ve had the opportunity to witness and appreciate the effort the collaboration as a whole has dedicated to fostering both scientific excellence and a sense of community.”
Chewin’ on muons
Muons resemble electrons, but are approximately 200 times more massive. Similar to electrons, muons possess a property known as spin, which arises from quantum physics and can be regarded as a tiny internal magnet. When exposed to an external magnetic field, that internal magnet will oscillate or precess, akin to the handle of a spinning top or the tip of a football thrown in an imperfect spiral.
The rate at which a muon precesses within a given magnetic field is described by a number referred to as the g-factor. Roughly a century ago, the value of g was predicted to be precisely 2. However, experimental observations soon indicated that g deviated slightly from 2 by a quantity recognized as the muon’s magnetic anomaly.
Researchers have formulated theories that account for that discrepancy, which has significant implications for humanity’s most comprehensive theory of the universe’s fundamental forces and particles, termed the Standard Model of particle physics.
“For over a century, g-2 has been enlightening us about the essence of nature,” remarked Lawrence Gibbons, a professor at Cornell University and analysis co-coordinator for the latest findings. “It’s thrilling to add a precise measurement that I believe will be relevant for a long time.”
To clarify, scientists understand that the Standard Model requires revision, Chupp indicated, but the specifics remain enigmatic. Given the intimate connection between the muon magnetic anomaly and the Standard Model, it became a natural avenue for inquiry.
“As it has been for decades, the magnetic moment of the muon continues to serve as a stringent benchmark of the Standard Model,” stated Simon Corrodi, assistant physicist at Argonne National Laboratory and analysis co-coordinator. “The new experimental result illuminates this fundamental theory and will establish the standard for any new theoretical calculations to follow.”
The more accurately scientists can quantify g-2, the more assured they can be in piecing together the puzzles presented by the Standard Model in its current iteration.