Muon g-2 Particle Accelerator Experiment Results Are Not Explained by Our Current Theories of Physics

Muon g-2 Experiment at Fermilab

The Muon g-2 ring sits in its detector hall amidst electronics racks, the muon beamline, and other equipment. Credit: Reidar Hahn, Fermilab

The first results from the Muon g-2 experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory have revealed that fundamental particles called muons behave in a way that is not predicted by scientists’ best theory to date, the Standard Model of particle physics. This landmark result, published recently in the journal Physical Review Letters, 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. The muons in this experiment act as a window into the subatomic world and could be interacting with yet-undiscovered particles or forces.

“This experiment is a bit like a detective story,” said team member David Hertzog, a University of Washington professor of physics and a founding spokesperson of the experiment. “We have analyzed data from the Muon g-2’s inaugural run at Fermilab, and discovered that the Standard Model alone cannot explain what we’ve found. Something else, perhaps beyond the Standard Model, may be required.”

The Muon g-2 experiment is an international collaboration between Fermilab in Illinois and more than 200 scientists from 35 institutions in seven countries. UW scientists have been an integral part of the team through the Precision Muon Physics Group — constructing sensitive instruments and sensors for the experiment, and leading data analysis endeavors. In addition to Hertzog, current UW faculty and lead scientists involved include Peter Kammel, research professor of physics; Erik Swanson, a research engineer with the UW’s Center for Experimental Nuclear Physics and Astrophysics, or CENPA; Jarek Kaspar, a research scientist; and Alejandro Garcia, a professor of physics.

Lead Fluoride Crystals

Lead fluoride crystals, which are used in detectors designed and constructed at the UW that measure muon decay products for the Muon g-2 experiment. Credit: University of Washington

“The UW custom-built instrumentation would not have been possible without the extraordinary dedication and expertise of our CENPA technical staff, who work closely with our postdocs and graduate students,” said Hertzog.

A muon is about 200 times as massive as its cousin, the electron. They occur naturally when cosmic rays strike Earth’s atmosphere. 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. The strength of the internal magnet determines the rate that the muon precesses in an external magnetic field and is described by a number known as 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 slightly. The Standard Model predicts with high precision what the value of this so-called “anomalous magnetic moment” should be. 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.

Hertzog, then at the University of Illinois, was one of the lead scientists on the predecessor experiment at Brookhaven National Laboratory. That endeavor concluded in 2001 and 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:

  • g-factor: 2.00233183620(86)
  • anomalous magnetic moment: 0.00116591810(43)

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 theoretical predictions at a significance of 4.2 sigma, a little shy of the 5 sigma — or 5 standard deviations — that scientists prefer as a claim of discovery. But it is still compelling evidence of new physics. The chance that the results are a statistical fluctuation is about 1 in 40,000.

“This result from the first run of the Fermilab Muon g-2 experiment is arguably the most highly anticipated result in particle physics over the last years,” said Martin Hoferichter, an assistant professor at the University of Bern and member of the theory collaboration that predicted the Standard Model value. “After almost a decade, it is great to see this huge effort finally coming to fruition.”

Erik Swanson

UW research engineer Erik Swanson with equipment used to measure magnetic fields in the Muon g-2 experiment. Credit: University of Washington

The Fermilab experiment, which is ongoing, reuses the main component from the Brookhaven experiment, a 50-foot-diameter (15-meter-diameter) superconducting magnetic storage ring. In 2013, it was transported 3,200 miles (5,150 kilometers) 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 “wobbling.”

Many of the sensors and detectors at Fermilab were constructed at the UW, such as instruments to measure the muon beam as it enters the storage ring and to detect the telltale particles that arise when muons decay. Dozens of scientists — including faculty, postdoctoral researchers, technicians, graduate students, and undergraduate students — have worked to assemble these sensitive instruments at the UW and then install and monitor them at Fermilab.

UW scientists have also been involved in theoretical work around the Muon g-2 collaboration.

“The prospects of the new result triggered a coordinated theory effort to provide our experimental colleagues with a robust, consensus Standard-Model prediction,” said Hoferichter, who was a UW research assistant professor from 2015 to 2019. “Future runs will motivate further improvements, to allow for a conclusive statement if physics beyond the Standard Model is lurking in the anomalous magnetic moment of the muon.”

In its first year of operation, in 2018, the Fermilab experiment collected more data than all prior muon g-factor experiments combined. The Muon g-2 collaboration has now finished analyzing the motion of more than 8 billion muons from that first run. The UW team was central to this effort, leading to four doctoral theses to date.

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,” said Fermilab scientist Chris Polly, who is a co-spokesperson for the current experiment and was a lead University of Illinois graduate student under Hertzog during the Brookhaven experiment. “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.”

“With these exciting results our team, in particular our students, is enthusiastic to push hard on the remaining data analysis and future data-taking in order to realize our ultimate precision goal,” said Kammel.

For more on this research:

Reference: “Measurement of the Positive Muon Anomalous Magnetic Moment to 0.46 ppm” by B. Abi et al. (Muon g−2 Collaboration), 7 April 2021, Physical Review Letters.
DOI: 10.1103/PhysRevLett.126.141801

9 Comments on "Muon g-2 Particle Accelerator Experiment Results Are Not Explained by Our Current Theories of Physics"

  1. Bibhutibhusanpatel | April 27, 2021 at 10:33 pm | Reply

    This is true that a new force is present.Fact can be fòund from 4.1 sigma result clearly without error,except to get too small more accurate value.Ñow problem is that what this new force is beyond magnetism presenting in the form of extra.

  2. Bibhutibhusanpatel | April 27, 2021 at 10:41 pm | Reply

    A fifth force is discivered has littĺe doubt.Now problem is thet,what this extra magnetism is.

    • Torbjörn Larsson | April 29, 2021 at 12:47 pm | Reply

      There is all doubt, see my own response and its links. This will be a long race until it’s concluded either way

    • Torbjörn Larsson | April 29, 2021 at 12:54 pm | Reply

      I just saw an experimentalist’s take on this:

      “So, in about two years it would not be unreasonable to have the next result, which would be quite a bit more precise because it combines runs two and three. Then there will be another run, and we will probably finish taking data in another two years or so. The precise end of measurements is still somewhat uncertain, but I would say that about five years from now, maybe sooner, we should have a very clear picture.”

      [ ]

      The new theory method may take a year or two to check out, and several years to improve as well [see the Forbes link article in my response to the article, and its figure of that theory progress on estimates].

  3. It took you three weeks to write an article about the g2 muon controversy yet you haven’t included a single word about the QCD correction that, if correct, means the results of the g2 experiment actually *confirms* the standard model instead of challenging it? The correction that was published in Nature literally the day after the muon g2 experiment results were published? There’s no excuse for this, any in depth research of the topic would have brought this to your attention.

    You deliberately chose to leave it out so you can inspire fake optimism in your readers that hopelessly cling to the idea that techgurus and science will be able to pull us out of the coming dark ages. All you do is further pull the veil over out heads by giving us a reality that’s simple and inoffensive. You’re part of the poison that’s killing our society.

    • Torbjörn Larsson | April 29, 2021 at 12:46 pm | Reply

      Give the interested science readers a break.

      If you knew how to read an article you would note that it is the original press release that either didn’t know or didn’t care about the simultaneous release of a competing theoretical value.

      And if you cared about science and/or other reades would also link to that release instead of just claiming it exists. You were correct, but a happenstance reader has only your word for it.

      And seeing how you then go into a baseless conspiracy theory – which thus is guaranteed to be incorrect, because what would be the chances – your entire comment becomes suspect. To be sophistic, but also factual: You’re part of the poison that’s needlessly worsening our society.

  4. Torbjörn Larsson | April 29, 2021 at 12:40 pm | Reply

    Notably there was a simultaneous release involving a new theoretical method that handles the strong force more comprehensively and that match the new experimental values. That method needs to be repeated in its latest, more precise form but earlier results have tended to cover the experimental regime – see the figure in an article that summarizes the events:

    “While the Muon g-2 team is justifiably celebrating this momentous result, this discrepancy between two different methods of predicting the Standard Model’s expected value — one of which agrees with experiment and one of which does not — needs to be resolved before any conclusions about “new physics” can responsibly be drawn.”

  5. … we really need a priest…

  6. Y’all opening the doors to HeLL those are demons being released God save us from Satan workers please in Jesus name🙏

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