York U researchers calculate most precise measurement of helium

Researchers at York University have achieved the most precise measurement of the fine structure of helium ever recorded.

Eric Hessels

“It’s important because there could be clues to some of the mysteries of physics lurking inside of these measurements, things like dark matter,” said Distinguished Research Professor Eric Hessels in the Department of Physics and Astronomy, Faculty of Science.

Hessels and his graduate students Koskuke Kato and Taylor Skinner worked for eight years on the measurement.

They were measuring the energy difference between two different, but closely related, orbits of the helium atom, known as the helium fine structure intervals.

“If you can measure these intervals in helium, you can determine the strength of the electric force between any two charged particles,” said Hessels. “It measures a very fundamental constant of physics, the fine structure constant, which is important for understanding all electric and magnetic forces.”

Hessels, Kato and Skinner measured the fine structure to nine digits of accuracy. Hessels credits their success in part to the use of a new technique he and a former postdoctoral Fellow, Amar Vutha, developed in his lab a few years earlier – the frequency offset separated oscillatory fields (FOSOF) technique. This technique is a modification of the separated oscillatory fields technique that has been around for almost 70 years and won Norman F. Ramsey a Nobel Prize.

Hessels believes their modified technique will become an important tool in the field of physics going forward.

“Quite a few people around the world have been trying to measure the fine structure of helium, but no one else was using this technique,” said Hessels. “The FOSOF technique led to a better way to determine the energy difference and, ultimately, a major improvement in precision for helium fine-structure measurements.”

Hessels and his team were also able to use more atoms by concentrating them using lasers. Laser excitation and ionization allowed them to detect these atoms more readily.

Hessels and his postdoctoral fellow came up with the idea of FOSOF a few years earlier, but this is the first time they used it to make a precision measurement, and it worked.

The results, “Ultrahigh-Precision Measurement of the n=2 Triplet P Fine Structure of Atomic Helium Using Frequency-Offset Separated Oscillatory Fields,” were published in the journal Physical Review Letters, where it became an editors’ suggested paper.

Hessels also recently completed a measurement of the size of a proton and is now turning his attention to measuring the electron's electric dipole moment.

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