Muon g – 2: A benchmark study challenges the rules of particle physics

The newly published results of the international experiment suggest the possibility of a new physics that rules the laws of nature, scientists say. The results of the experiment, which studied a subatomic particle called a muon, do not match the predictions of the Standard Model, on which all particle physics is based, and instead reaffirm the discrepancy discovered in the experiment 20 years ago. In other words, the physics we know cannot alone explain the measured results. The study was published in the journal Physical Review Letters.

What is a standard model?

The standard model is a rigorous theory that predicts the behavior of the building blocks of the universe. It sets out the rules for six types of quarks, six leptons, the Higgs boson, three basic forces, and how subatomic particles behave under the influence of electromagnetic forces.

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Moon is one of the leptons. It is similar to an electron, but 200 times larger and much more unstable, surviving a split second. The experiment, called Muon g – 2 (g minus two), was conducted at the National Fermi Acceleration Laboratory at the U.S. Department of Energy (Fermilab).

What was this experiment about?

He measured the amount related to muon, following a previous experiment at Brookhaven National Laboratory, under the U.S. Department of Energy. Concluded in 2001, Brookhaven’s experiment yielded results that did not coincide identically with the predictions of the Standard Model.

The Muon g – 2 experiment measured this amount with greater precision. Efforts were made to find out whether the discrepancy would continue or whether the new results would be closer to predictions. It turned out that there was a deviation again, albeit a minor one.

What quantity was measured?

It is called the g-factor, a measure that results from the magnetic properties of muons. Because the muon is unstable, scientists are studying the effect it leaves behind on its environment.

Muons behave as if they have a tiny inner magnet. In a strong magnetic field, the direction of this magnet is “climate”, just like the axis of a sinkhole. The rate of oscillation of muons is described by the g-factor, the magnitude measured. It is known that this value is close to 2, so scientists measure a deviation of 2. Hence the name g – 2.

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The G-factor can be precisely calculated using a standard model. In the g – 2 experiment, scientists measured it with high – precision instruments. They generated muons and made them orbit a large magnet. The moons also interacted with the “quantum foam” of subatomic particles that “pops up and disappears”, as Fermilab described it. These interactions affect the value of the g-factor, causing the muons to nod slightly faster or somewhat slower. The magnitude of this deviation (this is called the anomalous magnetic moment) can also be calculated using the Standard Model. But if the quantum foam contains additional forces or particles that the standard does not take into account, it would further adjust the g-factor.

What are the findings?

The results, while different from the predictions of the Standard Model, strongly agree with the results from Brookhaven, Fermilab said.

Accepted theoretical values ​​for muon are:
g-factor: 2.00233183620
anomalous magnetic moment: 0.00116591810

The new experimental results (combined from the Brookhaven and Fermilab results) released on Wednesday are:
g-factor: 2.00233184122
anomalous magnetic moment: 0.00116592061.

What it means?

The results of Brookhaven and now Fermilab suggest the existence of unknown interactions between muons and magnetic fields – interactions that could involve new particles or forces. However, it is not the last word in paving the way for new physics.

To claim the discovery, scientists need results that differ from the Standard Model by 5 standard deviations. The combined results of Fermilab and Brookhaven differ by 4.2 standard deviations. While this may not be enough, it is unlikely to be a coincidence – that chance is about 1 in 40,000, according to a press release from the Argonne National Laboratory, also under the U.S. Department of Energy.

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