The Muon g-2 experiment has announced its third, final and most precise result for the magnetic anomaly of the muon, a value that sheds light on fundamental forces and particles at work in the universe
An international team of scientists, including physicists from the University of Michigan, has made the most precise measurement ever of an important, wobbly feature of a subatomic particle called the muon.
Working on the Muon g-2 experiment, hosted by the U.S. Department of Energy's Fermi National Accelerator Laboratory, the team released its third and final measurement of the muon's magnetic anomaly.
"This is a really important place to look for nature to reveal to us secrets we haven't yet uncovered," said Timothy Chupp, a professor of physics who leads the U-M team's contributions to the Muon g-2 experiment. "Measuring this magnetic property with this precision and comparing it to theory is how we can explore new kinds of interactions that affect the evolution of the universe and everything around us."
The muon magnetic anomaly, also called g-2 (pronounced "gee minus two"), is the experiment's namesake. This value is the result of the measurement of the wobble of the muon's magnetic pole and the strength of the magnetic field. Mathematically, the magnetic anomaly is denoted am and is equal to g-2 divided by 2.
With this new and final measurement, the collaboration builds on the precision it reported for the g-2 value in 2021 and 2023. This new, final result agrees with those results, but with a much better precision that even surpasses the original design goal of the experiment.
Precision is like golf in that a lower score indicates superior performance. In its 2023 release, the Muon g-2 team reported a precision of 200 parts per billion. With its latest results, that has improved to 127 parts per billion, surpassing the experiment's design goal of 140 parts per billion.
"This is a very exciting moment because we not only achieved our goals but exceeded them, which is not very easy for these precision measurements," said Peter Winter, a physicist at Argonne National Laboratory and co-spokesperson for the Muon g-2 collaboration. "With the support of the funding agencies and the host lab, Fermilab, it has been very successful overall, as we reached or surpassed pretty much all the items that we were aiming for."
The measured value of g-2 may or may not hint at new physics, such as undiscovered particles. That determination will have to be made by the scientists working to calculate what the theoretical value of g-2 should be. Currently, there are two approaches that yield different numbers.
The second, more recent number does push the needle away from needing new physics to explain the results. As theorists continue to work, though, the Muon g-2 Experiment has done its job in providing a sufficiently precise experimental value for them to compare against.
"It's an exciting result and great to see an experiment come to a definitive end with a precision result," 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 results June 3 and has submitted the paper reporting the result to the journal Physical Review Letters.
Michigan's magnetic moment
The Muon g-2 collaboration is made up of nearly 176 scientists from 34 institutions in seven countries. The geographic breadth of the experiment was matched by the technical breadth of its group members. That's not always the case for large experiments, but it was key to the success of Muon g-2, said co-spokesperson Marco Incagli, a physicist with the Italian National Institute for Nuclear Physics at Pisa.
"This experiment is quite peculiar because it has very different ingredients in it," Incagli said. "It is really done by a collaboration among communities that normally work on different experiments."
David Aguillard, a graduate student working with Chupp at U-M, agreed.
"Measuring a fundamental property of the muon to the precision and accuracy the g-2 collaboration has achieved is a monumental effort drawing on many areas of expertise," Aguillard said. His work helped verify Muon g-2's magnetic field measurement, which is one of two experimental values required to determine g-2, he said.
"My work is one of many cross-checks that go beyond the scope of the measurement itself, which are necessary to ensure the accuracy of the final result," Aguillard said.
Another area where the U-M team contributed was led by Eva Kraegeloh, a graduate student completing a dual doctorate in physics and scientific computing. She led the analysis that combined maps of the magnetic field and the muons' locations in the ring, which let researchers understand how muons felt the magnetic field in Muon g-2's 50-foot ring.
"It's been very rewarding to work with so many talented people on a shared goal, not only to push the science forward, but also on an interpersonal level," Kraegeloh said. "I got to see and appreciate the effort that the collaboration as a whole has put into fostering both scientific excellence and a sense of community."
Chewin' on muons
Muons are similar to electrons, but about 200 times more massive. Like electrons, muons have a property called spin, which arises from quantum physics and can be thought of as a tiny internal magnet. In the presence of an external magnetic field, that internal magnet will wobble, or precess, like the handle of a spinning top or the point of a football thrown in a less-than-perfect spiral.
How fast a muon precesses in a given magnetic field is described by a number called the g-factor. Roughly 100 years ago, the value of g was predicted to be exactly 2. But experimental measurements soon showed g to be slightly different from 2 by a quantity known as the magnetic anomaly of the muon.
Scientists have developed theories that account for that difference, which has big implications for humanity's most comprehensive theory of the universe's fundamental forces and particles, dubbed the Standard Model of particle physics.
"For over a century, g-2 has been teaching us a lot about the nature of nature," said Lawrence Gibbons, professor at Cornell University and analysis co-coordinator for the latest result. "It's exciting to add a precise measurement that I think will stand for a long time."
To be clear, scientists know the Standard Model needs to be revised, Chupp said, but exactly how is a mystery. Given the muon magnetic anomaly's intimate connection to the Standard Model, it became a natural place to look for answers.
"As it has been for decades, the magnetic moment of the muon continues to be a stringent benchmark of the Standard Model," said Simon Corrodi, assistant physicist at Argonne National Laboratory and analysis co-coordinator. "The new experimental result sheds new light on this fundamental theory and will set the benchmark for any new theoretical calculation to come."
The more precisely scientists can measure g-2, the more confident they can be in their approach to piecing together puzzles presented by the Standard Model in its current form.
