Doug Bonderud

Oct 18th 2021

Meet the Muon: Wobbling Its Way Into Particle Physics History


Particle physics is on a quest to uncover the fundamental components of the universe by analyzing, cataloging and measuring the particles that combine to create everything. This is no easy task — the sheer variety and variability of these physical firmaments makes this a constantly changing puzzle that can seem utterly inscrutable until a new secret is unlocked.

Now, new research into the muon — a small, electron-like particle 200 times heavier than its negatively charged cousin — may point the way to the next particle pivot: wobbles.

Standard Operating Procedure

To help make sense of the universe and how it operates, scientists created the Standard Model. This wasn’t an easy task, either. As SciTechDaily notes, while the electron was discovered in 1897, the last piece of the current framework — the Higgs boson — wasn’t confirmed until 2012.

The model provides a way to reconcile three of the four fundamental forces: electromagnetism, the strong nuclear force and the weak nuclear force. It also highlights the role of quarks and leptons as the building blocks of all matter, including the familiar trio of protons, neutrons and electrons. Work on the Standard Model has pinpointed that photons play a role in carrying electromagnetism, while the strong force relies on gluons to ensure atomic nuclei stability and the weak force leverages W and Z bosons to drive the powerful nuclear processes that keep stars burning for billions of years.

However, despite its utility, the Standard Model has a blind spot: gravity. While the model does a great job describing which particles underpin the other three fundamental forces, gravity is conspicuously absent. That’s because scientists have never been able to tie a specific particle to the creation and distribution of gravity. While it plays a critical role in the operation of our universe — from counterbalancing the expansive tendencies of nuclear reactions within our sun to keeping our feet firmly planted on planet Earth — this seemingly simple attractive operation has historically resisted any in-depth understanding.

But new muon measurements may change all that.

Cracking the Code

What’s important to remember about scientists — and particle physicists specifically — is that they’re never satisfied with the status quo. While the Standard Model provides a starting point for understanding the universe, obvious gaps in the current framework mean there’s more work to be done. In practice, this work is about finding novel approaches to potentially break the model by conducting experiments, taking measurements and then comparing the results to what the model predicts.

The challenge? For years, the model has been coming up with the right answers, much to the frustration of physicists who know there’s something missing. Repeated experiments using different particle approaches have churned out accurate measurement after accurate measurement — until now. Recent follow-up work on previous muon experiments using more advanced equipment suggests a crack in the current model and offers hope that scientists may have finally found the key to a new revolution in particle physics.

Meet the Muon

So, what exactly is a muon? Think electron, but heavier. Much heavier — 200 times heavier, in fact. As Science Daily notes, both muons and electrons “are essentially tiny magnets with their own magnetic field.” However, unlike electrons, muons are far less stable, only existing for a few millionths of a second before they decay. It’s also difficult to observe muons even during their brief time here, since the vacuum they occupy isn’t empty.

“It’s your cappuccino-foam version of the vacuum, where there’s virtual particles winking in and out of existence all the time,” Lawrence Gibbons, who led the Cornell team involved in the new research, told Science Daily. “And that turns out to affect the strength of the magnetic field of a muon.”

Thanks to work done at CERN in 1959, followed by more precise experiments in 1966 and 1969, and then another round at Brookhaven National Laboratory in 1999, researchers finally found something: disconnect between observed and predicted magnetic measurements when muons made their way into this vacuum. New efforts at Fermilab with more precise and advanced equipment have confirmed these findings — and may pave the way for a much more massive muon impact.

Let’s Get Physics-al

The work at CERN, Brookhaven and Fermilab all focus on the same thing: the g-2 value of muon, which represents the amount they “wobble” via vacuum interactions. As Fermilab particle physicist Jessica Esquivel notes, this wobble is called the precession frequency.

“When muons go into a magnetic field, they precess, or they spin like a spinning top,” she tells Vox.

To measure precession, the Fermilab efforts used a powerful particle accelerator — capable of creating 20 times more muons than those used previously at CERN — to shoot an intense beam of muons into highly sensitive detectors, which measured their precession frequency. As the Standard Model predicted, this precession occurs when muons collide with virtual particles, which Esquivel describes as “sort of like ghosts of actual particles.”

“We have photons that kind of pop in and out and they’re just kind of like there, but not really there,” she told Vox. However, despite this here-and-gone-again strangeness, these virtual particles have a physical impact on muon wobbles.

But this is where it gets really weird: Experiments at Fermilab confirm that muons are wobbling more than they should be, according to the Standard Model. Even more exciting? Scientists don’t know why.

Wobbling Our Way Forward

The lack of certainty here is what makes these experimental results so interesting. Under the Standard Model, muon g-2 values are explainable using current particle interactions and should produce predictable results. However, the Fermilab efforts suggest that something else is causing this extra wobble — something that lies outside the current functional framework, as Esquivel and her colleagues told Vox.

And while there’s already been some discussion of this “breaking” the Standard Model, it’s more like filling in the details where data was clearly missing. Esquivel likens it to the addition of elements to the periodic table.

“Even back then,” she told Vox, “they had spots where they knew an element should go, but they hadn’t been able to see it yet. That’s essentially where we’re at now.” In practice, this muon movement opens the door to a host of potential particle operations. This unexpected wobble might be caused by interaction with long-predicted dark matter or dark energy particles, or it could help to establish a particle-based link to the outlying fourth fundamental force of gravity.

Esquivel puts the impact of this motion measurement simply: “It’s once-in-a-lifetime. We’re chasing new physics and we’re so close, we can taste it.” One muon meal, extra wobble — coming right up!

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