The long-awaited first results from the Muon g-2 experiment at the US Department of Energy’s Fermi National Accelerator Laboratory show basic particles called muons that behave in a way not predicted by scientists’ best theory, the standard model of particle physics. This milestone result, made with unprecedented precision, confirms a discrepancy that has been gnawing at researchers for decades.
The strong evidence that muons deviate from the calculation of the standard model may indicate that there is exciting new physics. Muons act as a window into the subatomic world and could interact with as yet undiscovered particles or forces.
“Today is an extraordinary day, long awaited not only by us but by the entire international physics community,”
A muon is about 200 times as massive as its cousin, the electron. Muons occur naturally when cosmic rays hit the Earth’s atmosphere, and particle accelerators at Fermilab can produce them in large numbers. Like electrons, muons act as if they have a small inner magnet. In a strong magnetic field, the direction of the muon’s magnet is forward or wobbled, as is the axis of a rotating top or gyroscope. The strength of the internal magnet determines the velocity that the muon needs 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.
When the muons circulate in the Muon g-2 magnet, they also interact with a quantum foam of subatomic particles jumping in and out of existence. Interactions with these short-lived particles affect the value of the g-factor, causing the muons’ precession to accelerate or decelerate very easily. The standard model predicts this so-called irregular magnetic moment extremely accurately. However, if the quantum foam contains additional forces or particles that are not taken into account in the standard model, it would further adjust the muon g factor.
“This quantity we measure reflects the interaction between muon and everything else in the universe. But when theorists calculate the same amount using all the known forces and particles in the standard model, we do not get the same answer, ”said Renee Fatemi, a physicist at the University of Kentucky and head of simulation for Muon. g-2 experiment. “This is strong evidence that the muon is sensitive to something that is not in our best theory.”
The predecessor experiment at DOE’s Brookhaven National Laboratory, completed 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 differs from the theory with the most accurate measurement to date.
The accepted theoretical values for the muon are:
g-factor: 2.00233183620 (86)
anomalous magnetic moment: 0.00116591810 (43)
[uncertainty in parentheses]
The new experimental world average results announced by the Muon g-2 collaboration today are:
g-factor: 2.00233184122 (82)
anomalous magnetic moment: 0.00116592061 (41)
The combined results from Fermilab and Brookhaven show a difference with the theory with a significance of 4.2 sigma, slightly shy of the 5 sigma (or standard deviations) that researchers need to require a discovery, but still convincing evidence for new physics . The chance that the results are a statistical fluctuation is approx. 1 out of 40,000.
The Fermilab experiment recycles the main component of the Brookhaven experiment, a 50-foot superconducting magnetic storage ring. In 2013, it was transported 3,200 miles ashore and by sea from the Long Island to 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, the 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, circulating thousands of times at almost the speed of light. Detectors located on the ring allow scientists to determine how fast the muons are moving.
In the first year of operation, in 2018, the Fermilab experiment collected more data than all previous muon-g factor experiments combined. With more than 200 researchers from 35 institutions in seven countries, the Muon g-2 collaboration has now finished analyzing the movement of more than 8 billion muons from the first run.
“After the 20 years that have passed since the Brookhaven experiment ended, it is so gratifying to finally solve this mystery,” said Fermilab scientist Chris Polly, a spokesman for the current experiment and a graduate student at the Brookhaven experiment.
Data analysis on the second and third runs of the experiment is underway, the fourth run is underway, and a fifth run is planned. Combining the results from all five runs will give researchers an even more accurate measurement of muons chess and reveal with greater certainty whether new physics is hiding in quantum foam.
“So far, we have analyzed less than 6% of the data that the experiment will eventually collect. Although these first results tell us that there is an exciting difference with the standard model, we will learn a lot more in the next few years, ”said Polly.
“Capturing the subtle behavior of muons is a remarkable achievement that will lead the search for physics beyond the standard model for many years to come,” said Fermilab’s deputy director of research, Joe Lykken. “This is an exciting time for particle physics research, and Fermilab is at the forefront.”
A press conference discussing the first results of the Muon g-2 experiment will be held at 12.00 US Central Time on April 7th. Journalists should contact email@example.com for information on the connection. More photos of the Muon g-2 experiment are available in the Muon g-2 gallery. More information about the experiment can be found at Muon g-2 site.
More ways to get involved: Take a virtual 360 tour of the Muon g-2 experiment, or watch a guided video tour. Watch the full Muon g-2 video playlist. Sign up for a free virtual public lecture on April 17 explaining the new Muon g-2 results. Print your own “Marvelous Muon” poster.
The Muon g-2 experiment is supported by the Department of Energy (US); National Science Foundation (US); National Institute of Nuclear Physics (Italy); Council for Science and Technology Facilities (UK); Royal Society (UK); European Union Horizon 2020; National Natural Science Foundation of China; MSIP, NRF and IBS-R017-D1 (Republic of Korea); and the German Research Foundation (DFG).
Fermilab is America’s leading national laboratory for particle physics research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and is operated under contract by Fermi Research Alliance LLC. Visit Fermilab’s website at https://www.fnal.gov and follow us on Twitter @Fermilab.
The DOE Office of Science is the largest single supporter of basic science research in the United States and is working to tackle some of the most pressing challenges of our time. For more information, visit https://science.energy.gov.