Northwestern physicists hold one antimatter particle still for months, to ask why matter won

You cannot keep antimatter in a jar. The moment it touches ordinary matter, both vanish in a flash of energy. So when a team at Northwestern University in the US wanted to study a single particle of it, they had to build a cage with no walls.

It worked. In a paper published on 21 October in Physical Review Letters, the physicists report that their antimatter trap caught one positron and held it still, in a carefully controlled quantum state, for months. “If you want to hold on to antimatter, you have to hold it in a container without walls,” Gerald Gabrielse, who led the work, told Scientific American.

Why it matters: theory says the Big Bang should have made equal amounts of matter and antimatter, which would have destroyed each other. Yet here we are, in a universe made almost entirely of matter. Nobody knows why.

How the antimatter trap slowed a racing positron to near stillness

What is a positron? It is the antimatter twin of the electron: the same mass, but a positive charge instead of a negative one.

The team, led by Gabrielse, director of Northwestern’s Center for Fundamental Physics, took its positrons from radioactive sodium-22. They fly out very fast, so the researchers sent them through a cold sliver of tungsten crystal to slow them down. Most were lost on the way, destroyed on contact with electrons. A few came out slow enough to be steered into the trap.

That answers a question many people search: how is antimatter stored? With electric and magnetic fields, in a near-perfect vacuum, so the particle floats in empty space and never touches a single wall. Physicists call this a Penning trap. Northwestern’s trap is cooled to about 300 millikelvin, just 0.3 degrees above absolute zero, cold enough for the positron to settle into its lowest quantum state.

Then came the delicate part. With tuned microwaves, the team nudged the positron between single energy levels and detected each change. They call the setup a “one-positron quantum cyclotron”. The first author is PhD student Thomas Myers, and the US National Science Foundation funded the work.

The real test has not happened yet

This is a tool, not yet a discovery. The goal is to measure the positron’s magnetic moment, roughly how strong a tiny magnet it is, and compare it with the electron’s. Gabrielse’s group has already measured the electron’s to about one part in 10 trillion, one of the most precise measurements ever made of a single particle.

Today’s best theory, the Standard Model, says the two should match exactly, with opposite signs. “If it turns out they are not equal, that would be the biggest discovery in physics in the last century,” Gabrielse said in Northwestern’s release.

Roni Harnik, a theoretical physicist at Fermilab who was not involved, called it “an exciting development that will enable us to test a basic symmetry,” in comments to Scientific American.

The team has not given a date for that measurement, or how precise it will be. Those are the numbers to wait for.

A question about everything, asked in one small lab

Will this change your phone or your power bill? No. This is basic physics, with no near-term use claimed. Nor is it dangerous. “We don’t have enough antimatter to make an explosion,” Gabrielse said.

But the question it chases belongs to everyone: why is there more matter than antimatter, and so why is there anything at all?

And positrons are less exotic than they sound. The “P” in a hospital PET scan stands for positron. According to the US National Institutes of Health, the scanner picks up energy released when positrons meet electrons in the body. In our reading, basic research often travels that way, from strange to useful, over decades. The quantum computing work behind Google’s Willow chip is another example still on that road.

Why a quiet lab result deserves attention

On balance, this looks like a meaningful step, earned the slow way. Holding one antimatter particle steady for months, and reading its quantum state without losing it, is a problem Gabrielse says his group chased for many years. In our reading, the group’s electron result suggests it can make the comparison that matters.

The honest caution is that this is not yet a new antimatter discovery. Current theory, as Gabrielse notes, predicts the two numbers will match. That would still be useful, closing one door where new physics could hide. Either way, the question moves forward.

The next thing to watch is the first positron magnetism figure from this trap. If it matches the electron, physics holds. If it doesn’t, the universe has some explaining to do.