This particle which seems to disobey the known laws of physics

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The much-anticipated results of an experiment have arrived, and they could be about to revolutionize physics as we know it.

They are sometimes called “big electrons”. Muons are indeed similar to their better-known cousins. In contrast, they’re 200 times heavier and radioactively unstable – decaying in just a millionth of a second into electrons and tiny, ghostly, charge-less particles called neutrinos. Muons also develop a property called “spin”. Concretely, they behave like tiny magnets, flickering like small gyroscopes when immersed in a magnetic field.

Predictions and results do not match

In quantum mechanics, the non-intuitive rules underlying the atomic realm, empty space is not really. In reality, it is bubbling with “virtual” particles entering and leaving existence. This entourage influences the behavior of existing particles, including a property of the muon: its magnetic moment, represented in the equations by a factor called g.

According to a formula derived in 1928 by Paul Dirac, founder of quantum theory, the factor g of a solitary muon should be 2. Now, as we have just said, muons are not alone. So the formula must be corrected to consider the quantum hum coming from all the other potential particles in the Universe. This causes the muon’s g-factor to be greater than 2.

In a recent highly anticipated experiment at Fermilab in Illinois (USA), dubbed Muon g-2, an international team of 200 physicists from seven countries, led by Dr Polly, fired muons through an intense magnetic field. What this team announces to us today is that the muons did not behave as expected, wobbling much more than calculations predicted.

In other words, this tiny subatomic particle seems to disobey the known laws of physics. These results are also consistent with those of similar experiments conducted at Brookhaven National Laboratory in 2001, which have teased physicists ever since.

Note that Fermilab researchers are also convinced that these results were not the result of statistical fluke. Their confidence threshold is indeed fixed at 4.2 sigma, which is incredibly close to the 5 sigma threshold which no longer raises any doubts. For information, a result of 5 sigma suggests that there is 1 in 3.5 million chance that a result obtained is the product of chance.

National Fermi Accelerator Laboratory, or Fermilab, in Batavia, Ill., Where the research is being conducted. Credits: Reidar Hahn / Fermilab, via US Department of Energy

A standard model tested

For decades, physicists have relied on the Standard Model, the series of equations that lists the fundamental particles in the Universe (17 at last count) and defines how they interact. But if this model has successfully explained the results of many experiments on high-energy particles, it does not explain EVERYTHING. Certain deep questions about the Universe indeed remain unanswered.

This new experience proves it again. The behavior of muons does pose a major new challenge to this model. “This is solid proof that the muon is sensitive to something that does not fit our best theory”, abstract thus Renee Fatemi, physicist at the University of Kentucky. The big question is therefore: what are these forms of matter and energy vital for the nature and evolution of the cosmos which are not yet known to science?

These results will soon be published in a series of articles submitted to several peer-reviewed journals. Others will also be communicated. This new work represents only 6% of the total data that this muon experiment is expected to collect in the years to come.

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