Physicists cage antimatter particle in quantum trap breakthrough

Physicists at Northwestern University have captured a positron, the antimatter twin of the electron, cooled it to its lowest possible energy state and held it for months. The team, led by Gerald Gabrielse, suspended the antimatter particle in mid-air using only electric and magnetic fields, building a device described in Nature as a new route to one of the sharpest tests in physics.

The test addresses a basic unanswered question: why is the universe made of matter at all? The Standard Model, the reigning theory of particles and forces, predicts the Big Bang should have produced equal amounts of matter and antimatter. When matter and antimatter meet, they annihilate into a burst of energy. Had they been produced in perfect balance, "all the matter and antimatter would annihilate and there'd be no Universe left," Gabrielse said. Something tipped the balance toward matter, and physicists still do not know what it was.

The search for a difference between matter and antimatter has been running since the 1960s. Researchers have found a slight preference for matter in the behavior of a handful of particles, but nowhere near enough to account for the universe's composition. Detailed comparisons of protons and antiprotons have revealed no discrepancy. The Northwestern group is taking a different approach: comparing the magnetic moment, which governs how a particle wobbles on its axis in a magnetic field, of electrons and positrons. That property can be measured 10,000 times more precisely in electrons and positrons than the equivalent property in protons, making it a much sharper test. Reported by phys.org, the work was published in Physical Review Letters on 21 October.

How to catch something that vanishes on contact

A positron annihilates the moment it touches ordinary matter, so the team could not use any container with solid walls. Instead, they started with fast positrons emitted by the radioactive decay of sodium-22, which emerge carrying energies as high as 546 kilo-electron-volts. The researchers slowed them by scattering them off atoms in a tungsten crystal. Many were destroyed in the process, and the team skimmed off the slowest survivors and guided them into the trap through a 625-micrometer opening.

Some of the surviving positrons briefly bound with electrons to form neutral positronium, which could pass into the trap; the electric field then pulled each pair apart and retained the positron. Loading each particle takes eight to eleven hours. Inside, the positron floats in a near-perfect vacuum in a Penning trap chilled to 300 millikelvin, barely above absolute zero, which removes stray gas molecules and lets the particle survive for months.

The team then demonstrated control over the positron's individual quantum states, according to the study. Tuned microwave pulses added precise amounts of energy, and the researchers detected each quantum jump as a shift in the particle's oscillation frequency, also flipping its spin. "We cool it down so much that it's in its quantum ground state," Gabrielse said. "We actually coaxed this very fast positron to sit there and behave for months at a time."

Why the measurement matters

The trapping and control are preparation for the experiment's goal: measuring the positron's magnetic moment and comparing it with the electron's. Gabrielse's group previously measured the electron's magnetic moment to about a part in ten trillion, among the most precise measurements ever made of an elementary particle. Matching that precision with antimatter would test CPT symmetry, the Standard Model principle that matter and antimatter are perfect mirror images, more than thirty times more precisely than has ever been possible for leptons.

"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 experiment is "a remarkable achievement" that will provide "a powerful probe for physics beyond the standard model," said Barbara Maria Latacz, an antimatter physicist at CERN. "I cannot wait to see their first precision measurements," added Stefan Ulmer of Heinrich Heine University Düsseldorf.

The work was supported by the National Science Foundation under grant 2110565, part of a continuing wave of public investment in basic research such as Canada's push to fund the next generation of researchers. The precision magnetic-moment measurement itself has not yet been made; the paper demonstrates the trapping and quantum-state control needed to attempt it. Whether the positron's uncertainties can be brought into line with the electron's remains open, but the result gives physicists their clearest path yet to the most sensitive matter-antimatter comparison ever performed, joining other fast-moving frontiers in space science from the solved mystery of the Martian clouds to NASA's accelerated Moon return plans.