Northwestern Physicists Trap a Single Positron for a Month

Every ordinary particle of matter has an antimatter counterpart with equal mass and opposite charge. To build the trap, the physicists began with atoms of sodium 22, a radioactive isotope with a half-life of about two and a half years. When one of those atoms decays, it emits a positron, the antimatter counterpart of the electron. “The big challenge is to persuade these positrons to slow down and then capture them,” said Gerald Gabrielse, a Northwestern physicist who oversaw the project.

The first step is to send the positrons through the crystal lattice of a sliver of cold tungsten. As the particles scatter through the material, they slow down and lose energy. “Every time they bang into something, there’s a chance of annihilation, but it doesn’t always happen,” Gabrielse said. “If they make it to the end, they get popped out.”

The physicists then steer the positrons along a channel using electromagnetic fields. The particles eventually enter a vacuum just three tenths of a degree Celsius above absolute zero, held by an electric field above and below and a magnetic field on the sides. “If you want to hold on to antimatter, you have to hold it in a container without walls,” Gabrielse said. The method allowed the researchers to isolate and study a single, lone positron for a month; the report says it could let them do so indefinitely.

The team intends to use the trap to measure the positron’s magnetic moment, meaning the strength and orientation of its magnetic field. That requires determining the energy needed to flip the direction of the positron’s spin and the energy needed to make it jump from its ground state to its first excited state. With a similar trap setup, Gabrielse and his colleagues have already measured the electron’s magnetic moment to a precision of one part in 10 trillion. The corresponding positron measurement would allow a direct comparison.

“We’re looking to see if they have an exactly opposite magnetic moment,” Gabrielse said. That is the prediction of the Standard Model of particle physics. If the values differ, the report says, the result could help explain why the universe is made of matter instead of antimatter.

Editor's Summary

Northwestern physicists trapped a single positron for a month using a wall-less electromagnetic trap, enabling precision measurements of its quantum properties. The team plans to compare the positron’s magnetic moment with the electron’s, a test of the Standard Model that could bear on the matter-antimatter imbalance. The result was reported in Physical Review Letters.