Small particles create an even bigger hole in our understanding of reality and make the physicists dance


The Muon g-2 experiment at FermilabImage Fermilab

Around half past five Dutch time, a wave of excitement spread through the international community of particle physicists. The American Fermilab, the famous particle institute in Chicago, then announced that they had exposed muons – heavier variants of electrons – to a powerful magnetic field. When the particles began to spin and wobble like a spinning top on a table, they did so in a way that was unmistakably slightly different from the best theoretical predictions. The most likely explanation? The influence of an unknown force of nature or new fundamental particles.

The measurement can thus cause a true natural science evolution. ‘I literally danced through the room of enthusiasm,’ says particle physicist Tristan du Pree of the Dutch particle institute Nikhef, who was not involved in the experiment herself. Together with his wife – also a particle physicist – he followed the livestream of the announcement with beer and pizza. ‘Fortunately it turned out to be a wonderful result. We’ve been waiting for this for twenty years. ‘

Muons again

The announcement follows two weeks after other big news from the world of particle physics. At the time, researchers from the European particle lab Cern also shared abnormal behavior in muons, although this involved different behavior.

‘It is no coincidence that this happens twice in muons,’ says Du Pree. Behind the scenes, the same force or particle could pull the strings. “But the honest answer is, we don’t know yet.”

The new results are important because they mean that the laws that describe the behavior of our physical reality at the finest level may be different than thought.

A scientist at work on the so-called Muon g-2 experiment.  Image Fermilab

A scientist at work on the so-called Muon g-2 experiment.Image Fermilab

Coffee mug or T-shirt

This concerns the behavior of particles, the building blocks of which everything from slippers to complete galaxies are made. Decades ago, physicists combined all known particles and their mutual interactions in the so-called standard model of particle physics, which fits on a coffee mug or T-shirt. At the same time, physicists know that there must be more than that standard model. For example, gravity, which glues us to the earth and keeps the earth in orbit around the sun, is still not understood in the model.

That is why people have been eagerly looking forward to the result from Fermilab for years. Physicists had already seen the deviation from the model in another experiment in 2001, but the underlying statistic was not strong enough at the time. The hope was that more measurements would place the find on firmer ground. That turned out to be the case on Wednesday afternoon.

One in 40 thousand

The probability that the reported deviation from the standard model is nothing more than a statistical mirage has decreased to roughly 1 in 40 thousand after the most recent result. This means that the result is also stronger than what the European colleagues reported two weeks earlier. In their experiment, that chance was 1 in 1,000.

Both results do not reach the gold standard that particle physicists use to be able to speak of a discovery. Then that chance must decrease to 1 in 3.5 million. ‘It’s only a matter of time’, says Du Pree. ‘In the current analysis, they have only analyzed 6 percent of the measurement data. So there is still more than enough in the pipeline to overcome that last hurdle. ‘

Complex calculation

Incidentally, a pressing issue still hangs over the market. The prediction for the twirling and twirling of the muons follows from a long, complex calculation. ‘Whether there may be a mistake in this: that is a question that will be asked more often in the coming years,’ says Du Pree.

In spite of this, the Standard Model – and the accompanying understanding of the physical reality in which we live – seems to be showing increasingly emphatic cracks. We have to wait who will give the last push.

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