Electron magnetism confirms the most accurate prediction of particle physics

No one has tried this piece too harshly.

In a new experiment, scientists have measured the magnetic property of an electron more precisely than ever before, with the most accurate property of any elementary particle ever measured. Known as the electron’s magnetic moment, it is a measure of the strength of the magnetic field carried by the particle.

That property is predicted by a particular standard model of physics, a theory that describes particles and forces at the subatomic level. In fact, it is precisely the prediction made by that system.

By comparing the new ultraprecise measurement and prediction, the scientists gave the theory one of its best tests yet. The new measurement agrees with the standard model’s prediction of about 1 part per trillion, or 0.1 billionths of a percent, scientists report on February 17. Physical Review Letters.

When a theory makes a prediction with great precision, it’s like a physicist’s Bat Signal, calling for researchers to test it. “Some of us are resistant,” says Gerald Gabriel, a physicist at Northwestern University in Evanston, Ill.

To measure the magnetic moment, Gabrielse and colleagues studied a single electron for months on end, capturing it in close proximity to a magnet and how it responded when covered with microwaves. The team determined the electron’s magnetic moment to 0.13 parts per trillion, or 0.0000000001 percent.

A demanding measure is a complex task. “It’s such a challenge that no one but Gabriel’s team dares to do it,” says physicist Holger Müller of the University of California, Berkeley.

The new result is more than twice as accurate as the previous measurement, which stood for over 14 years, and which was also made by the Gabrielse machine. Now the researchers have finally won. “When I saw [paper] “Wow, they did it,” says Stefano Laporta, a theoretical physicist affiliated with the University of Patavina in Italy, who works on calculating the electron’s magnetic moment based on the model.

The new test of the standard would be more impressive if the conundrum was not in another precise measurement. Two recent experiments, one conducted by the physicist Saïda Guellati-Khélifa of the Kastler Brossel Laboratory in Paris, the other by Müller, disagree on the value of a number called the subtle constant, which characterizes the strength of electromagnetic interactions (SN: 4/12/18). That number is input to the prediction of the magnetic resonance electron model. Thus, the discrepancy limits the precision of the new test. If that discrepancy were sorted out, the test would be done exactly 10 times as it is now.