First results from Fermilab's Muon g-2 experiment strengthen evidence of new physics

Fermilab and Illinois Physics
4/7/2021

UIUC scientists play leading role in paradigm-shifting experiment

The first results from the Muon g-2 experiment hosted at Fermi National Accelerator Laboratory show fundamental particles called muons behaving in a way not predicted by the standard model of particle physics. These results confirm an earlier experiment of the same name performed at Brookhaven National Laboratory. Combined, the two results show strong evidence that our best theoretical model of the subatomic world is incomplete. One potential explanation would be the existence of undiscovered particles or forces.

The Muon g-2 ring sits in its detector hall amidst electronics racks, the muon beamline, and other equipment. This impressive experiment operates at negative 450 degrees Fahrenheit and studies the precession (or wobble) of muons as they travel through the magnetic field. Photo by Reidar Hahn/Fermilab
The Muon g-2 ring sits in its detector hall amidst electronics racks, the muon beamline, and other equipment. This impressive experiment operates at negative 450 degrees Fahrenheit and studies the precession (or wobble) of muons as they travel through the magnetic field. Photo by Reidar Hahn/Fermilab
The long-awaited first results from the Muon g-2 experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory show fundamental particles called muons behaving in a way that is not predicted by scientists’ best theory, the Standard model of particle physics. This landmark result, made with unprecedented precision, confirms a discrepancy that has been gnawing at researchers for decades.

Physicists at the University of Illinois Urbana-Champaign played a major role in producing the new result, which confirms the result from two decades ago: muons behave in a way that is not predicted by scientists’ best theory, the Standard model of particle physics.

“It was extremely exciting to produce such a tantalizing result,” said Paul Debevec, UIUC emeritus physics professor and member of the Brookhaven experiment. “We knew that we were going to have to do the experiment again—and do it better. What we didn’t know at the time was that it would take us 20 years to get to a new result.”

Two decades ago, an experiment at Brookhaven National Lab, which also included several University of Illinois physicists, announced that the muon didn’t behave quite as expected when placed in a magnetic field. Could the unexpected behavior be telling us something new about the universe? Could there be something wrong with the measurement? Or the expectation?

The current experiment found strong evidence that muons deviate from the Standard model calculation, which might hint at exciting new physics. Muons act as a window into the subatomic world and could be interacting with yet undiscovered particles or forces. The new result was presented today by the Muon g-2 experiment (pronounced mew’-on g minus two) at Fermilab.

“We have been working on this experiment for many years, painstakingly acquiring the data and making sure that our results are correct,” said Esra Barlas Yucel, UIUC postdoctoral researcher on the experiment. “It’s exciting to share this news with the world.”

“When I first started working on this experiment, I couldn’t imagine how we were going to produce a measurement with a precision of 0.5 parts per million. It’s truly unbelievable. We were successful through the teamwork of a lot of dedicated scientists,” said Cristina Schlesier, a UIUC graduate student who is part of the Muon g-2 team.

A muon is about 200 times as massive as its cousin, the electron. Muons occur naturally when cosmic rays strike Earth’s atmosphere, and particle accelerators at Fermilab can produce them in large numbers. Like electrons, muons act as if they have a tiny internal magnet. In a strong magnetic field, the direction of this magnet precesses, or wobbles, much like a spinning top or gyroscope. The strength of the internal magnet determines the rate that the muon precesses in an external magnetic field and is described by a number that physicists call the g-factor. This number can be calculated with ultra-high precision.

As the muons travel through the experiment, they also interact with a quantum foam of subatomic particles popping in and out of existence. Interactions with these short-lived particles affect the value of the g-factor, causing the muons’ precession to speed up or slow down very slightly. The Standard model predicts this so-called anomalous magnetic moment extremely precisely. But if the quantum foam contains additional forces or particles not accounted for by the Standard model, that would tweak the muon g-factor further.

“Our measurement tells us how the muon interacts with everything else in the universe,” said Sudeshna Ganguly, a UIUC postdoctoral research on this project who recently transitioned to a scientific position at Fermilab. “When the theorists calculate the same quantity, using all of the known forces and particles in the Standard model, we don’t get the same answer. That might mean that this result is telling us about a part of the universe that we have not yet observed.”

The calculation of the muon’s behavior is just as important as the measurement. This is another place where UIUC scientists have played a key role.

“When this experiment was proposed, one of the major questions was whether or not we could calculate the theoretical expectation with high enough precision,” said Aida El Khadra, UIUC physics professor and co-chair of the Muon g-2 Theory Initiative, a worldwide effort to bring scientists together to produce the most precise calculation possible. “We’ve made tremendous progress in recent years, and believe we will continue to improve the precision of the theoretical calculation as the experimental precision improves. This result is the first step of several years of exciting science.”

The predecessor experiment at Brookhaven, which concluded in 2001, offered hints that muons’ behavior disagreed with the standard model. The new measurement from the Muon g-2 experiment at Fermilab strongly agrees with the value found at Brookhaven and diverges from theory with the most precise measurement to date.

The accepted theoretical values for the muon are:

g-factor: 2.00233183620 ± 0.00000000086

anomalous magnetic moment: 0.00116591810 ± 0.00000000043

The new experimental world-average results announced by the Muon g-2 collaboration today are:

g-factor: 2.00233184122 ± 0.00000000082

anomalous magnetic moment: 0.00116592061 ± 0.00000000041

The combined results from Fermilab and Brookhaven have a significance of 4.2 sigma, a little shy of the 5 sigma (or standard deviations) that scientists require to claim a discovery but still compelling evidence of new physics. The chance that the results are a statistical fluctuation is 1 in 40,000.

The Fermilab experiment reuses the main component from the Brookhaven experiment, a 50-foot-diameter superconducting magnetic storage ring. In 2013, it was transported 3,200 miles by land and sea from Long Island to the Chicago suburbs, where scientists could take advantage of Fermilab’s particle accelerator and produce the most intense beam of muons in the United States. Over the next four years, researchers assembled the experiment; tuned and calibrated an incredibly uniform magnetic field; developed new techniques, instrumentation, and simulations; and thoroughly tested the entire system.

The Muon g-2 experiment sends a beam of muons into the storage ring, where they circulate thousands of times at nearly the speed of light. Detectors lining the ring allow scientists to determine how fast the muons are precessing.

In its first year of operation, in 2018, the Fermilab experiment collected more data than all prior muon g-factor experiments combined. With more than 200 scientists from 35 institutions in seven countries, the Muon g-2 collaboration has now finished analyzing the motion of more than 8 billion muons from that first run.

“After the 20 years that have passed since the Brookhaven experiment ended, it is so gratifying to finally be resolving this mystery,” said Fermilab scientist Chris Polly, who is a co-spokesperson for the current experiment and was a University of Illinois graduate student on the Brookhaven experiment.

Data analysis on the second and third runs of the experiment is under way, the fourth run is ongoing, and a fifth run is planned. Combining the results from all five runs will give scientists an even more precise measurement of the muon’s wobble, revealing with greater certainty whether new physics is hiding within the quantum foam.

 “So far we have analyzed less than 6 percent of the data that the experiment will eventually collect. Although these first results are telling us that there is an intriguing difference with the Standard model, we will learn much more in the next couple of years,” said Adam Schreckenberger, UIUC postdoc on the experiment.

“Pinning down the subtle behavior of muons is a remarkable achievement that will guide the search for physics beyond the Standard model for years to come,” said UIUC Physics Professor and Fermilab Chief Research Officer Kevin Pitts. “This is an exciting time for particle physics research, and Fermilab is at the forefront.”

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