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Why some physicists are skeptical about the muon experiment that hints at “new


One of the smallest things in the universe could have just changed everything we know about it. 

On Wednesday, the U.S. Department of Energy’s Fermi National Accelerator Laboratory (Fermilab) in Illinois revealed much-anticipated results from a storied particle physics experiment known as Muon g-2. The bizarre results, which showed something quite different than what standard theories projected, shocked physicists around the world — and, if confirmed, suggest that fundamental physics theories may be wrong.

“This is our Mars rover landing moment,” Fermilab physicist Chris Polly told the New York Times of the findings. 

The data, published in the journal Physical Review Letters, showed that fundamental particles called muons behaved in a way that was not predicted by the Standard Model of particle physics. The Standard Model is a gold standard theory that explains the four known forces in the universe and all fundamental particles. The Standard Model even predicted the existence of the Higgs Boson decades before it was experimentally detected in 2012.

“This is strong evidence that the muon is sensitive to something that is not in our best theory,” Renee Fatemi, a physicist at the University of Kentucky and the simulation manager for the Muon g-2 experiment, said in a press statement.

The aforementioned particles, known as muons, acted peculiarly when exposed to a strong magnetic field at Fermilab. That odd result could be the result of a new, as-yet-undiscovered fundamental particle — which could potentially throw a wrench in everything humans know about physics.

But not all physicists buy the results. The reason has to do, in part, with a number called sigma.

Seeking sigma

In physics, as in most sciences that involve experiments, one’s experimental results are characterized by a number, sigma, that relays how likely it is that said result is random chance.

Say that you penned a theory that said that coins would always come up heads, and then performed an experiment in which you flipped a coin 10 times and saw that your coin came up heads every time. It is actually possible that this could happen — in fact, about one in a thousand times, it will — but your results, though initially shocking, would not cause a rethinking of the theory of coin flipping. That’s because 10 flips is not enough trials to warrant a sigma number that would signify “true without a shred of doubt” status.  That would require a so-called 5 sigma result, which corresponds to a probability of one in 30 million that your experiment was a fluke.

The Fermilab experiment with muons was a follow-up to an experiment at Brookhaven National Laboratory in 2001, which had a significance of about 3.7 sigma. Combined with the Fermilab’s results, the sigma value has increased to a 4.2 sigma; 5 is the golden standard for scientists to claim a new discovery.

In other words, the Muon g-2 experiment did not reach that golden standard five-sigma bar. 

Once in a blue muon

Despite being one of twelve fundamental particles in the universe, muons are rarely seen; they have properties similar to everyday electrons, in that they hold a charge, yet their mass is far greater than their electron cousins. Muons are also extremely shortly-lived: after they are created in high-energy collisions, such as when cosmic rays strike Earth’s atmosphere, they decay in an average of 1.56 microseconds later. It is one of physics’ great mysteries that some of the universe’s fundamental particles would be so ill-equipped to survive in this universe.

Similar to its cousin the electron, muons have an internal magnetism; like any magnet, they can be manipulated and redirected in the presence of magnetic fields. Particle accelerators at Fermilab can produce muons in large quantities, which is what researchers at Fermilab did for the Muon g-2 experiment — tracked how muons interact in a particle accelerator in the presence of a strong magnetic field.

In such a magnetic field, the muon wobbles in a manner determined by an intrinsic number known as the g-factor. This number changes depending on the muon’s environment and interactions with other particles. Muon g-2 is designed to measure the muon’s g-factor to a very high precision.

What happened at the Muon g-2 experiment is, quite simply, that the expected result different from what theory dictates. The discrepancy, on paper, appears tiny. According to the Standard Model, the accepted g-factor for the muon is 2.00233183620. But the new experiment yielded results at 2.00233184122 — a difference of 0.00000000502.

That might seem small. But for a theory that has accurately predicted the properties of particles to more digits than that, this discrepancy is huge.

“This quantity we…



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