A single antimatter particle was held in a trap for months

In a lab in Illinois, physicists captured and tamed a tiny particle of antimatter in a trap with invisible walls. By studying how the positron behaved as time goes by, the researchers may soon have a chance to break our best mathematical model of physical reality.

The positron is the antimatter counterpart of the electron, which means the two particles have the same mass but the opposite electric charge. Unlike electrons, however, positrons are rare and typically exist for a tiny fraction of a second because the moment one meets one of the super-abundant electrons in our universe, the pair annihilate. This makes positrons difficult to study.

Gerald Gabrielse at Northwestern University in Illinois and his colleagues have now found a way to trap a positron and keep it around for months on end. By analysing data collected while the positron was trapped, they plan to precisely measure the antiparticle’s properties and consequently investigate one of the biggest problems with the standard model of particle physics.

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The researchers obtained the positron from a radioactive isotope of sodium that emits one whenever it decays. Such positrons are very fast and hard to control, so they placed the sodium into a special refrigerator to lower its temperature and energy, and slow down the antiparticles.

They slowed them further by directing the positrons through a tungsten crystal. While some positrons were annihilated as they travelled through the crystal, some came through unscathed. They were then directed into a refrigerated chamber that was empty of all other particles. Here, electronic components generated electromagnetic fields that confined a single incoming positron, serving as invisible walls that trapped the particle.

The researchers then pushed the positron into its lowest possible energy state, which Gabrielse says allowed them to keep the otherwise elusive antiparticle trapped for months. They could also push it into the next lowest energy state with controlled bursts of microwaves.

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“It’s a real kick to stand in my lab and watch a positron making transitions – ‘quantum jumps’ we call them – between its lowest energy states. How much more fundamental, how much neater can it get?” he says.

“Trapping an antiparticle is extremely difficult,” says Stefan Ulmer at Japanese scientific institute RIKEN, particularly since any stray particle, burst of light or tiny vibration could disturb it and the trap had to be kept only tens of a degree warmer than the coldest temperature possible.

The payoff for trapping and controlling the positron despite these experimental challenges could be remarkable. “This has the potential to become one of the most precise measurements of a fundamental constant in physics,” says Ulmer.

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Specifically, Gabrielse and his colleagues want to measure the positron’s magnetic moment, which is a number that quantifies its response to magnetic fields. The researchers have previously made the most precise ever measurement of the electron’s magnetic moment, and comparing this figure with the one for the antiparticle would enable them to compare the properties of matter and antimatter with unprecedented precision.

The standard model predicts they should have identical magnetic moments, but this prediction underscores a notable shortcoming of this theory. The problem is that if matter and antimatter are so alike, then they should have been created in equal amounts and fully annihilated as the universe began, leaving nothing. “The standard model seems to be inconsistent, or at least significantly incomplete, with basic features of the universe, like why we have a universe at all,” says Gabrielse.

Uncovering a difference between the electron and positron could hint at some currently unknown mechanism that resulted in our matter-filled universe. If all goes according to plan, the researchers aim to have data that will shed light on this within the year.

Physical Review Letters DOI: 10.1103/gbkc-6nlj