The Standard Model of particle physics has passed one of the most severe tests


In a new experiment, scientists measured the magnetic properties of an electron more thoroughly than ever before, making the most precise measurements of any property of an elementary particle. Known as the electron’s magnetic moment, it is a measure of the strength of the magnetic field carried by the particle.

This property is predicted by the Standard Model of particle physics, a theory that describes particles and forces at the subatomic level. In fact, this is the most accurate prediction made by this theory.

By comparing the new ultra-precise measurement and the prediction, the scientists gave the theory one of its most rigorous tests. The new measurement agrees with the Standard Model prediction to within about 1 trillionth, or 0.1 billionth of a percent, the physicists report in Physical Review Letters from February 17.

When a theory makes a prediction with high accuracy, it’s like a physicist’s bat signal calling for researchers to test it. “For some of us, it’s overwhelming,” says physicist Gerald Gabriels of Northwestern University in Evanston, Illinois.

To measure the magnetic moment, Gabrielse and his colleagues spent months studying a single electron by trapping it in a magnetic field and watching it react to microwaves. The team determined the electron’s magnetic moment to be 0.13 parts per trillion, or 0.000000000013 percent.

Such a precise measurement is a difficult task. “It’s so difficult that no one but Gabrielse’s team dares to do it,” says physicist Holger Müller of the University of California, Berkeley.

A team of physicists (pictured) trapped a single electron (experimental equipment shown) and studied it for months to measure a property called the electron’s magnetic moment.LAURA NEVINS

The new result is more than twice as accurate as the previous measurements, which took more than 14 years and were also made by Gabrielse’s team. Now researchers have finally outdone themselves. “When I saw [статтю]I said, ‘Wow, they’ve done it,'” says Stefano Laporta, a theoretical physicist at the University of Padua in Italy who works on calculating the electron’s magnetic moment from the Standard Model.

The new test of the standard model would be even more impressive if not for a puzzle during another painstaking measurement. Two recent experiments, one led by physicist Sayda Guellati-Khelifa of Kastler Brossel’s laboratory in Paris and the other led by Muller, disagree on the value of a number called the fine-structure constant that characterizes the strength of electromagnetic interactions. This number is the input to the standard model for predicting the electron’s magnetic moment. Consequently, the discrepancies limit the accuracy of the new test. If this discrepancy were to be eliminated, the test would become 10 times more accurate than it is now.

The powerful standard model has withstood a barrage of experimental tests for decades. But scientists don’t think this is the be-all and end-all. This is partly because it does not explain observations such as the existence of dark matter, the invisible substance that exerts a gravitational influence on the cosmos. And it doesn’t say why the universe contains more matter than antimatter. Therefore, physicists continue to look for cases where the standard model breaks down.

One of the most tantalizing hints of the failure of the Standard Model is the magnetic moment, not of the electron, but of the muon, the electron’s heavy cousin. In 2021, a measurement of this property hinted at a possible discrepancy with the Standard Model predictions.

“Some people believe that this discrepancy could be a sign of new physics beyond the standard model,” says Guellati-Khelifa, who wrote a commentary on a new paper on the electron magnetic moment in the journal Physics . If so, any new physics that affects the muon can also affect the electron. Therefore, future measurements of the electron’s magnetic moment may also deviate from the prediction, finally revealing flaws in the standard model.

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