What is special about muons

Muon experiment: "I couldn't have dreamed it better"

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They arise when other particles collide, are considered to be the heavy cousins ​​of electrons and could shake the standard model of physics: muons. A measurement of these particles at the Fermilab in Chicago is causing excitement for women physicists around the world. What happened? The Dresden physicist Dominik Stöckinger was there and explains why this discovery is considered a sensation in the professional world.

ZEIT ONLINE: A "crack in the world model" writes Spektrum.de, the "farewell to the standard model" announced the FAZ. What kind of law of physics is that wavering with the discovery from Chicago?

Stockinger: In the world of physics, 17 particles are known to date, the basic building blocks of everything that exists in the cosmos. According to this standard model, muons - certain elementary particles - would have to rotate in a certain way in a magnetic field. But now an experiment in Chicago showed: They don't do that. What could have been a measurement error after a first experiment at Brookhaven National Lab (BNL) in 2001 now seems to be confirmed. And that means: There must be more elementary particles out there than physics previously thought.

ZEIT ONLINE: Let's talk about the experiment. In Chicago there is a particle accelerator in the Fermilab - similar to the LHC at Cern in Switzerland, which is well known here in Europe. He shoots a type of particle, a proton *, at a block of material to make other particles. What exactly is being researched there?

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Stockinger: At the Fermilab, we create muons with the particle accelerator and let them rotate in a magnetic field to see how they rotate. We measure how fast the particle rotates around itself and how fast it circles through the ring-shaped magnetic field storage ring. The relationship between the two rotations shows how the muon is influenced by the magnetic field. The Standard Model predicts exactly how the muons should behave. But the previous experiment at Brookhaven National Lab had already suggested that this movement is actually different.

ZEIT ONLINE: Electrons, neutrinos, quarks - they all belong to the elementary particles, the basic building blocks of matter. What are these muons that are causing so much eddy now?

Stockinger: In principle, muons are the heavy brothers of the electrons, the third sibling is called Tau. They belong to the same type of particle. Muons are rarely found in our environment because they decay after a few millionths of a second. On earth they are otherwise constantly created in the atmosphere when cosmic radiation occurs, i.e. light from space. Some * muons decay into other particles in the air before they reach the earth's surface. That's why we have to produce them specially for our experiments in the particle accelerator.

ZEIT ONLINE: Why do you do all the work and create muons artificially when similar electrons are everywhere on earth?

Stockinger: Muons are exactly the golden mean for our question: They are heavier than electrons, which is why the effects outside of the Standard Model have a stronger influence on their rotation. And compared to the even heavier rope, they are easier to manufacture and do not disintegrate quite as quickly.

ZEIT ONLINE: The result of the measurement on the Fermilab can be summarized in a simple number: a = 0.00116592061. The physical quantity called "anomaly of the magnetic moment" - for which a stands here - deviates from the prediction in the eighth decimal place. What's so sensational about that?

Stockinger: This is the first tangible evidence that the Standard Model cannot explain reality. There are already other effects that we can only explain if there are more particles than described in the theory, such as the neutrino mass. Or that dark matter even exists. These other effects simply do not occur in the standard model. In the muon experiment, we have the special situation that you can calculate exactly what value the model predicts: and this is exactly what you can then check in the experiment. The deviation shows us: The Standard Model does not explain everything that happens to the muon.