Physicists Trap and Cool a Single Positron to Study Matter–Antimatter Differences

Physicists have suspended a single positron—the antimatter equivalent of an electron—in the air and cooled it to its lowest possible energy state.

The new setup will allow researchers to study the magnetic properties of positrons with unprecedented precision. The team hopes to identify subtle differences between matter and antimatter and shed light on one of physics’ greatest mysteries: why the Universe is made overwhelmingly of matter.

Why is there more matter than antimatter?

The Standard Model, the best current theory of particles and forces, suggests that matter and antimatter should have been produced in equal amounts during the Big Bang.

Matter and antimatter have the same mass but opposite properties, such as electric charge. When they meet, they annihilate each other in an explosion of energy. If equal amounts had been produced and survived, “all matter and antimatter would have disappeared and there would be no universe,” says Gerald Gabrielse, a physicist at Northwestern University in Evanston, Illinois, who led the study.

Finding a difference between matter and antimatter in these measurements “would be revolutionary in physics,” Gabrielse says. Despite its known limitations, the Standard Model has proved remarkably accurate at predicting experimental results. For example, it cannot explain the apparent existence of dark matter, which is thought to interact primarily through gravity.

Details of the experiment were published in Physical Review Letters on 21 October.¹ Barbara Maria Ratacze, an antimatter physicist at CERN near Geneva, Switzerland, called the work a “remarkable achievement” that provides “a powerful exploration of physics beyond the Standard Model.” Stefan Ulmer, a physicist at Heinrich-Heine University of Düsseldorf in Germany, said he was looking forward to seeing the team’s first precise measurements.

Searching for differences between matter and antimatter

Since the 1960s, physicists have studied antimatter in increasing detail, looking for contradictions between the behavior of matter and antimatter. They have observed a slight preference for matter in the behavior of a small number of particles, but not enough to explain the composition of the Universe.

Highly detailed comparisons of protons and antiprotons have so far revealed no differences. Gabrielse’s group now plans to compare a property that can be measured 10,000 times more precisely in electrons and positrons: their magnetic moment. This property determines how a particle behaves as it spins in a magnetic field.

How researchers captured a single positron

The team first had to overcome the challenge of capturing positrons. Researchers began with fast-moving positrons produced by radioactive decay and slowed them by scattering them from atoms in a tungsten crystal.

Many positrons were annihilated during the process, but the researchers selected the slowest survivors and transferred them into a trap. The resulting one-positron quantum cyclotron made it possible to suspend and cool a single particle for highly precise measurements.