Learn what wobbling muons say about particles in the universe
On Saturday, April 17, the Fermilab Office of Education & Public Engagement will host a special virtual event, "What Can Wobbling Muons Tell Us About the Particles in Our Universe? The First Result from the Muon g-2 Experiment" with Dr. Adam Lyon at 7:30 p.m. (CST). It is free but registration is required at events.fnal.gov/arts-lecture-series/.
On Wednesday, April 7, the Muon g-2 Experiment at Fermilab released its eagerly awaited first result. In the experiment, muons (like electrons but heavier) race around the 150-foot circumference magnetic racetrack, wobbling as they go like tops slowly spinning on their axes.
Quantum mechanics allows for "virtual" subatomic particles to ever so briefly come in and out of existence and affect the wobble of muons. The Fermilab experiment measures this wobbling with more precision than ever before.
The previous incarnation of this experiment, performed two decades ago at Brookhaven National Laboratory on Long Island, N.Y., measured a small discrepancy from the theoretical prediction and hints that muons were influenced by perhaps undiscovered particles.
Their hint was not strong enough to claim discovery but compelled Fermilab to perform the experiment again with many improvements.
After years of planning, an amazing land and sea journey of the magnetic ring, assembly, testing, and operations, they have the first result from the data. Will they confirm or refute the Brookhaven discrepancy? In the April 17 virtual event, Lyon will discuss that and what these wobbling muons in a magnet can tell us about the particles that make up the universe.
Adam Lyon is a senior scientist and associate division head at Fermilab. He specializes in computing for experimental particle physics -- both traditional classical computing and more recently quantum computing. He joined the Fermilab Muon g-2 experiment at its founding in 2011.
This talk will be presented virtually on the Zoom platform. Reserve your spot just as you would order a ticket online at events.fnal.gov. You will receive a receipt for this purchase and then contacted closer to the lecture date regarding the Zoom link.
More ways to engage
In addition to the free virtual public lecture on April 17 that will explain the new Muon g-2 results, you can take a virtual 360 tour of the Muon g-2 experiment at 360.fnal.gov/vr/muong2/muong-2.html. Watch a guided video tour on Fermilab's YouTube channel. View the full Muon g-2 video playlist.
Or print your own "Marvelous Muon" poster.
April 7 news release
The first results were released during an April 7 new conference on the Muon g-2 experiment at the U.S. Department of Energy's Fermi National Accelerator Laboratory showing 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.
The strong evidence that muons deviate from the Standard Model calculation 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.
"Today is an extraordinary day, long awaited not only by us but by the whole international physics community," stated Graziano Venanzoni, co-spokesperson of the Muon g-2 experiment and physicist at the Italian National Institute for Nuclear Physics, in an April 7 news release. "A large amount of credit goes to our young researchers who, with their talent, ideas and enthusiasm, have allowed us to achieve this incredible result."
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 the muon's magnet precesses, or wobbles, much like the axis of 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 circulate in the Muon g-2 magnet, 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.
"This quantity we measure reflects the interactions of the muon with everything else in the universe. But 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," stated Renee Fatemi, a physicist at the University of Kentucky and the simulations manager for the 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, which concluded in 2001, offered hints that the muon's 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:
anomalous magnetic moment: 0.00116591810(43)
[uncertainty in parentheses]
The new experimental world-average results announced by the Muon g-2 collaboration today are:
anomalous magnetic moment: 0.00116592061(41)
The combined results from Fermilab and Brookhaven show a difference with theory at 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 about 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," stated Fermilab scientist Chris Polly, who is a co-spokesperson for the current experiment and was a lead 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% 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," Polly said in the release.
"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 Fermilab Deputy Director of Research Joe Lykken. "This is an exciting time for particle physics research, and Fermilab is at the forefront."
More information about the experiment is available at the Muon g-2 website at muon-g-2.fnal.gov.
The Muon g-2 experiment is supported by the Department of Energy (U.S.); National Science Foundation (U.S.); Istituto Nazionale di Fisica Nucleare (Italy); Science and Technology Facilities Council (U.K.); Royal Society (U.K.); European Union's Horizon 2020; National Natural Science Foundation of China; MSIP, NRF and IBS-R017-D1 (Republic of Korea); and German Research Foundation (DFG).
Fermilab is America's premier national laboratory for particle physics research. A U.S. Department of Energy Office of Science laboratory, Fermilab is located near Chicago, Illinois, and operated under contract by the Fermi Research Alliance LLC. Visit Fermilab's website at www.fnal.gov and follow Fermilab on twitter.com/Fermilab.
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Fermi National Accelerator Laboratory is operated by Fermi Research Alliance, LLC for the US Department of Energy.