Northwestern physicists trap a single antimatter particle for months to test a basic law of physics

Physicists at Northwestern University have held a single particle of antimatter motionless and alone inside a vacuum chamber for months, suspending it with electric and magnetic fields at a fraction of a degree above absolute zero and steering it between precise quantum states one microwave pulse at a time. The work, published Oct. 21 in Physical Review Letters, lays the groundwork for one of the most exacting tests yet of whether matter and antimatter obey identical physical laws.

The particle is a positron, the antimatter counterpart of the electron. The Northwestern team, led by physicist Gerald Gabrielse and first-authored by doctoral student Thomas Myers, captured the positron from the radioactive decay of sodium-22 nuclei, slowed it, and guided it into a Penning trap — a chamber that confines a charged particle using electric and magnetic fields rather than physical walls, so the antimatter never touches ordinary matter and annihilates. Cooled to roughly 300 millikelvins, about three-tenths of a degree above absolute zero, the positron settled into its lowest-energy quantum state inside a 5.6-tesla magnetic field. From there, the researchers used carefully tuned microwaves to nudge it between discrete, fully resolved cyclotron and spin energy levels and recorded each transition — a configuration the team calls a "one-positron quantum cyclotron," described in a preprint on arXiv ahead of the PRL paper.

Why a lone antimatter particle matters

The point of trapping a single positron for months is not the trapping itself — physicists have held antiparticles in isolation for long stretches before — but the degree of quantum control it now allows. Gabrielse's group previously used an equivalent apparatus built around a single electron to measure the electron's magnetic moment, a measure of how the particle behaves in a magnetic field, to roughly one part in 10 trillion, among the most precise measurements of any property of any elementary particle. The positron's magnetic moment is currently known far less precisely. The Northwestern program, described on the lab's Center for Fundamental Physics research page, aims to measure it roughly 150 times more accurately than before, using the same one-particle quantum-cyclotron method now demonstrated for the positron.

The payoff would be a stringent test of a bedrock assumption of particle physics: CPT symmetry, which holds that applying three mirror-image transformations — swapping a particle for its antiparticle, reflecting space, and reversing time — should leave the laws of physics unchanged. One direct consequence is that an electron and a positron should have magnetic moments equal in size and opposite in sign, to the precision the Standard Model can predict. Confirming or breaking that equality in the lepton sector, the class of particles that includes electrons and positrons, would be one of the sharpest tests of the Standard Model ever attempted.

"If it turns out they are not equal, that would be the biggest discovery in physics in the last century."

That assessment came from Gabrielse, describing the stakes of the eventual comparison, in comments reported by Phys.org, which credited Northwestern University as the source of the account. He also noted that existing theory predicts no difference between the two measurements — meaning the experiment is a search for a crack in well-tested physics, not an expectation of finding one.

A long-standing mystery about the universe

The broader motivation traces to one of physics' unresolved puzzles: the universe is made almost entirely of matter, even though the Big Bang, under the Standard Model, should have produced equal amounts of matter and antimatter that would have annihilated each other completely, leaving behind only light. Something broke the symmetry between the two, and the Standard Model does not explain what. Comparing electron and positron properties with extreme precision is one of several experimental routes — alongside work at CERN's Antiproton Decelerator facility, where separate experiments trap antiprotons and antihydrogen atoms for related tests — that physicists use to hunt for any asymmetry that might point toward an answer.

No one outside the laboratory is directly affected by the new result, and the paper itself reports an experimental capability rather than a finished measurement of the positron's magnetic moment. The research was supported by the National Science Foundation, the John Templeton Foundation, the Department of Energy's Superconducting Quantum Materials and Systems Center, and the Masason Foundation, according to the Northwestern group's published acknowledgments.

What comes next

Demonstrating that a single positron can be held in a near-perfect vacuum for months and manipulated with the same quantum precision previously achieved only with electrons is, by the team's own account, the necessary first step rather than the final one. The actual high-precision measurement of the positron's magnetic moment — the comparison that would test CPT symmetry — is expected to take further years of data collection using the same apparatus, building on detection methods the group has spent two decades refining, including vacuum-noise-limited amplifiers and specially designed superconducting cavities developed with Fermilab collaborators. If the eventual measurement matches the electron's value within predicted limits, it will reinforce the Standard Model once more. If it does not, physicists say, it would be a result with ramifications far beyond one university physics lab.