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Author: Isabel Zhang

From: Chicago, IL, US

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On April 7, 2021, more than 200 physicists from seven countries gathered together on a Zoom call that revealed the fate of particle physics.

This team of physicists had spent three years collecting data for the Muon g-2 experiment: a much anticipated project headquartered at Fermilab (a particle physics and accelerator laboratory in Batavia, Illinois). The physicists had done their work partially in the dark, having a key variable hidden from them.

The experiment results were revealed on the call: muons did not behave as predicted when shot through an intense magnetic field at Fermilab. In essence, it disobeyed the law of physics. “This is our Mars rover landing moment,” said Chris Polly, a physicist at Fermilab who had been working toward this finding for most of his career. The physicists were left muonstruck.

Naturally, you probably didn’t get my joke. What are muons in the first place? It sounds cool that the law of physics may have just been broken… but what exactly can we do with this new information?

To understand what this is all about, let’s begin where it all started.

About the Experiment

The Muon g-2 experiment had aimed to determine, to the finest measurement, the strength of the internal magnetic field generated by a muon. A muon is a particle similar to an electron— but 200 times more massive and radioactively unstable. Muons are integral elements of the cosmos and constantly rain down on us as the indirect product of cosmic rays colliding with particles in Earth's atmosphere.

Scientists have made incredibly precise calculations of how muons should move and the strength of their magnetism. These calculations are part of the Standard Model of particle physics: the theory that describes three of the four known fundamental forces in the universe and classifies all known elementary particles.

To test this model, the Muon g-2 team of physicists watched muons as they wobbled in a magnetic field and clocked whether the wobble deviated from the theory’s predictions. And, indeed, it did!

These Muon g-2 experiment results actually agree with experiments at the Brookhaven National Laboratory in 2001 that had teased physicists ever since. The Brookhaven National Laboratory ran experiments with similar results, but didn’t have enough funding to repeat the experiment and back up the findings.

“After 20 years of people wondering about this mystery from Brookhaven, the headline of any news here is that we confirmed the Brookhaven experimental results,” said Dr. Polly from Fermilab.

What’s Next?

Researchers reported that the deviation of the Muon g-2 experiment from the Standard Model, which was 0.0000002 percent, was highly significant. In its press release, Fermilab even suggested that this could completely change our basic model of how subatomic particles work.

“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,” read the press release. In essence, since theory and experiment didn’t agree, there must be undiscovered particles or forces at work.

For decades, physicists have relied on and been bound by the Standard Model. However, the model leaves many deep questions about the universe unanswered. Many physicists believe that much new physics is yet to be found, if only they could see deeper and further.

The data from the Fermilab Muon g-2 experiment could be of major help to scientists eager to build the next generation of particle accelerators. It might also, in the future, lead to explanations for the kinds of cosmic mysteries that have long preoccupied scientists. What exactly is dark matter, the unseen stuff that astronomers say makes up one-quarter of the universe by mass? Why is there matter in the universe at all?

Marcela Carena, head of theoretical physics at Fermilab, said: “I’m very excited. I feel like this tiny wobble may shake the foundations of what we thought we knew.”

Indeed— this may not be the last time we’re all muonstruck.


About the Author: Isabel Zhang

Isabel is a senior in high school, and is interested in biology and engineering. In her free time, she loves to bake, sketch, and hang out with her family.



  1. A tiny particle’s wobble could upend the known Laws of Physics. (2021, April 10). Retrieved from

  2. Heffernan, V. (2021, May 18). To Observe the Muon Is to Experience Hints of Immortality. Retrieved from

  3. Overbye, D. (2021, April 07). A Tiny Particles Wobble Could Upend the Known Laws of Physics. Retrieved from

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