Physicists Trapped a Single Antimatter Particle for Months: Why It Could Rewrite Physics

Somewhere in a laboratory at Northwestern University, a single particle of antimatter has been floating in a trap for months — held in place by invisible electromagnetic walls, cooled to a fraction of a degree above absolute zero, and behaving, in the words of the physicist watching it, like an "artificial atom." The achievement, published in Physical Review Letters on October 21 and covered by Nature, is the first-ever "one-positron quantum cyclotron" — and it could become one of the most precise tests in all of physics.

The hardest catch in physics

A positron is the antimatter twin of the electron: same mass, opposite charge. The problem with studying one is that the moment antimatter touches ordinary matter, both annihilate in a flash of energy. You cannot put a positron in a bottle — touching the bottle would destroy it. As team leader Gerald Gabrielse told reporters, "If we want to study antimatter, we must handle it carefully." He added, reassuringly: "It's not dangerous. We don't have enough antimatter to make an explosion."

So the team built a container with no walls. They started with positrons flying out of the radioactive decay of sodium-22 at up to 88 percent of the speed of light, and slowed them by scattering them through a single crystal of tungsten. The slowest survivors were skimmed off, drifted as neutral positronium through a hole barely 625 micrometers wide, and were then captured by the trap's electric field — the electron escaping, the positron staying. Loading one particle this way takes eight to eleven hours. Once caught, it stayed for months, pinned at 300 millikelvins — just 0.3 degrees above absolute zero — in a near-perfect vacuum, free of stray molecules that could kill it.

Why one particle matters so much

The goal is to measure the positron's magnetic moment — the number that describes how the particle wobbles on its axis in a magnetic field. Gabrielse's group previously measured the electron's magnetic moment to one part in ten trillion, the most precise such measurement ever made. Doing the same for the positron would let physicists compare matter and antimatter with roughly ten thousand times the precision of current proton-antiproton tests — and, the team estimates, sharpen tests of a deep principle called CPT symmetry by more than thirty times.

Here is the paradox driving all of it. The standard model of particle physics predicts that matter and antimatter should be perfect mirror images — identical magnetic moments included. But if they were truly created in equal amounts at the birth of the universe, they should have completely annihilated, leaving nothing behind. There would be no stars, no planets, no us. "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," Gabrielse told New Scientist. Somewhere in the mirror, there must be a crack — and this lonely positron, jumping between its lowest quantum states in a frozen trap, may be where the crack finally shows.

"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," Gabrielse said. "How much more fundamental, how much neater can it get?" Physicists at CERN agree. As antimatter researcher Barbara Maria Latacz put it, the experiment is "a remarkable achievement" that will provide "a powerful probe for physics beyond the standard model." The precision measurements themselves are still ahead — but for the first time, the trap that makes them possible actually exists.

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