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The Muon g-2 experiment offers evidence of contradictions to physicists' current understanding of the natural world.

April 29, 2021

Cornell Researchers Aid in Discovery of Anomalous Muon Wobble, Challenging Understanding of Natural World

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The results of a recent experiment at Fermi National Accelerator Laboratory — the leading particle physics facility in the U.S. — suggests that our current understanding of the universe could be flawed, and Cornellians had a hand in the discovery.

The April 7 findings come from initial data collected during the Muon g-2 experiment, which measured the magnetic properties of a subatomic particle — the muon — to an unprecedented level of precision. The results contradict predictions based on the Standard Model, the current theoretical framework in particle physics, suggesting that our understanding of the universe remains incomplete.

Prof. Emeritus David Rubin, physics, and postdoctoral associate Kevin Labe, physics, were part of the Cornell team involved in the discovery.

To the average person, this discovery may not seem relevant — muons are generated naturally when cosmic rays enter the Earth’s atmosphere but have little impact on our daily lives.

Labe has a different perspective — he said that advances in our fundamental understanding of the universe fuel a wide spectrum of technological advances.

He pointed to the discovery of quantum mechanics, which led to the development of modern electronics.

“So it can be hard to see what’s going to come down the road from understanding things better, but generally, in history, it seems to be the case that those improvements… do come,” he said.

Muons are elementary particles — the basic building blocks of matter — that bear a negative charge. Labe described these particles as being similar to electrons, except much heavier and less stable.

The experiment at Fermilab measured the magnetic dipole moment of the muon. Rubin explained that this magnetic moment — which is the strength of the muon as a magnet — is a consequence of the spin of the muon, akin to the spin of a top.

Researchers have discovered that this measurement yields a result that differs from what the Standard Model predicts.

The name of the experiment, pronounced “gee minus two,” captures the essence of what the experimenters were searching for — the difference between the true and the most simply calculated values of the muon’s magnetic dipole moment. 

“According to the most basic theory — [which] would apply if the muon were all by itself in the universe — [the] g factor should be exactly equal to two,” Rubin said. “[We are] measuring the discrepancy between the real g factor and two. So we call it g-2.”

Since muons are affected by forces from the other particles around them, the difference between the actual g factor and the theoretical value can offer insight into what these other particles are and how they interact with muons. A discrepancy between these values could even suggest the existence of particles not described by the current underlying model of physics.

Labe explained that after combining previous data from Brookhaven National Laboratory with the new results from Fermilab, the measurements differ from the theoretical prediction by 4.2 standard deviations. However, this difference falls just short of the 5 standard deviations generally required in particle physics to classify a discovery.

Although the findings are not a definitive discovery, the deviation is still large enough to leave physicists speculating about what new phenomena remain undiscovered.

The muon g-2 experiments are also far from over — while Fermilab is currently in its third year of data collection, the reported results come from just one year of data. Researchers are hopeful that muon g-2 measurements will improve as more experimental data can be analyzed.

“Basically, as we average over more and more data, we will get more and more precise results,” Labe said.

According to Rubin, the experiment involves applying a magnetic field to muon particles and measuring their properties as they travel around a large ring. As the muon spins, its axis of rotation shifts due to the magnetic force, much like how a spinning top wobbles under the influence of gravity. Experimenters measured the rate at which this axis shifts, also known as the precession frequency.

By making the magnetic field highly uniform and measuring this rate of “wobble,” the experimenters were able to determine the magnetic moment very precisely, according to Rubin.

Cornell researchers played a key role developing the platform on which the experiments took place.

According to Rubin, the Cornell group designed and built a system called a magnetic kicker to steer the muons from where they are produced to the storage ring, where their properties can be measured as they revolve around the ring.

Rubin explained that the kicker magnet has to be turned on and then off after a very short time — a mere 150 billionths of a second — to ensure the muons travel along a consistent path.

The Cornell team also developed the electronic clock that enabled the precise timing measurements involved in determining the precession of the muons, according to Rubin.

All of these considerations allowed scientists to precisely calculate g-2 and increase their confidence that the actual value differs from theory. However, it is still unclear why the muon’s behavior seems to contradict the Standard Model.

“I think the easy answer is that we could hope that we could find corroborating evidence for some specific kinds of new particles at a particle accelerator,” Labe said.

The realm of possibilities, however, is somewhat limited, both by the size of the deviation as well as the success of the Standard Model in explaining many well-understood physical phenomena.

“Theorists all over the world are thinking up new physical constructs that could explain the discrepancy,” Rubin said. ”There [are] a lot of constraints because they want to find something that will explain this discrepancy but also doesn’t break everything else.” Rubin explained that the best theories that accomplish both of these goals will guide future experiments.