Antimatter trap holds a single positron for months

A new antimatter trap built at Northwestern University has held a single positron, the antimatter counterpart of the electron, in a controlled quantum state for months, the research team reported in the journal Physical Review Letters on Wednesday.

The experiment, led by physicist Gerald Gabrielse, coaxed a single particle of antimatter into what the team calls a "one-positron quantum cyclotron": an artificial atom whose quantum energy levels scientists can manipulate and observe. It is the first time a lone antimatter particle has been placed into such a state, according to Scientific American.

Antimatter is difficult to study because a positron carries the same mass as an electron but the opposite charge, and the instant it touches ordinary matter both vanish in a burst of energy. Gabrielse, the university's Board of Trustees Professor of Physics and director of its Center for Fundamental Physics, led the work with doctoral student Thomas Myers as first author.

Catching a particle that cannot touch anything

The team's positrons come from the radioactive decay of sodium-22, an unstable isotope that sheds them naturally. Getting one into the trap is a slow process. "The big challenge is to persuade these positrons to slow down and then capture them," Gabrielse said. Loading a single particle takes up to eleven hours.

First, the positrons are fired through a sliver of cold tungsten. They ricochet through the crystal lattice, shedding speed with each collision. "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."

Electromagnetic fields then steer the survivors into a near-perfect vacuum chilled to 300 millikelvins, just above absolute zero. An electric field caps the top and bottom while a magnetic field closes the sides, so the particle floats in a container with no physical walls. "If you want to hold on to antimatter, you have to hold it in a container without walls," Gabrielse said.

Inside the trap the positron circles in what physicists call cyclotron motion. The team cooled that motion to its quantum ground state, then used tuned microwave pulses to flip the particle's spin and nudge it between energy levels one jump at a time. Each transition could be detected without disturbing the state.

A test for the Standard Model

The point of the exercise is comparison. The Standard Model of particle physics predicts that an electron and a positron should behave as near-perfect mirror images. Measuring the positron's magnetic moment with extreme precision and setting it beside the electron's would test that prediction more strictly than ever before.

Gabrielse's group is prepared for the comparison. In earlier work the team used a one-electron quantum cyclotron to measure the electron's magnetic moment to about one part in 10 trillion, among the most precise measurements ever made of an elementary particle. The new trap opens a path to the same kind of measurement for antimatter.

That measurement has not been made yet. The current paper demonstrates the control needed to attempt it: months of stable confinement, resolved quantum states, and clean detection of individual transitions. The team, which received support from the National Science Foundation, described the advance in a Northwestern news release.

The symmetry matters because the Big Bang should have produced matter and antimatter in equal amounts, yet the universe is made almost entirely of matter. Known differences in how particles and antiparticles decay, called CP violation, are not large enough to explain the imbalance, and physicists count it among the deepest open questions in the field. A mismatch between the electron and positron magnetic moments would crack the Standard Model and supply a new clue.

"Current theories suggest that there will be no difference in the two measurements," Gabrielse said. "If it turns out they are not equal, that would be the biggest discovery in physics in the last century."

The result arrives in a busy season for science news. Just this week, solar activity raised hopes that the northern lights could return this weekend after a solar wind surge, while new research examined an antioxidant lung cancer link across new studies. For Gabrielse's team, the next step is the measurement the trap was built for.