Physicists Trap Single Antimatter Particle for Months to Probe Universe’s ‘Greatest Loophole’
Physicists have successfully captured and maintained a single particle of antimatter—a positron—for several months, a technological breakthrough that opens a new window into why the universe exists. Led by Gerald Gabrielse, the research team at Northwestern University achieved this feat by isolating the particle in a specialized “trap with no walls,” allowing for the most precise measurements of antimatter ever attempted.
The experiment is part of a decades-long effort to identify flaws in the Standard Model of physics. While the Standard Model accurately describes three of the four fundamental forces—electromagnetism, the strong nuclear force, and the weak nuclear force—as well as the fundamental particles that compose the universe, it contains a massive logical gap. According to the theory, the Big Bang should have produced equal amounts of matter and antimatter, which would have promptly annihilated each other, leaving behind an empty universe of pure energy. Instead, the observable universe is composed almost entirely of matter, a discrepancy Gabrielse calls a “loophole” in current scientific understanding.
To investigate this imbalance, Gabrielse’s team turned to the positron, the antimatter counterpart of the electron. In the world of physics, positrons and electrons are identical in every way except for their charge. However, studying them is notoriously difficult because any contact between a positron and its matter counterpart results in immediate annihilation, releasing a burst of energy. This characteristic makes conventional storage impossible; a positron cannot touch the sides of a physical container without being destroyed.
The research team overcame this by utilizing a state-of-the-art trap that uses a combination of precise electric and magnetic fields to suspend the positron in the center of an empty vacuum. By ensuring the particle never touches the physical walls of the apparatus, the researchers were able to “tame” the particle, keeping it stable for months at a time. Gabrielse noted that while the process sounds volatile, the amount of antimatter involved is too small to be dangerous or cause a significant explosion.
Capture and stabilization required a complex sequence of energy reduction. The positrons were sourced from the radioactive decay of sodium-22, a rare isotope. When positrons emerge from this decay, they possess immense kinetic energy. To catch them, the team had to dramatically slow the particles before guiding them into a near-perfect vacuum. Once inside, the interior of the trap was cooled to approximately 300 millikelvins—a mere 0.3 degrees above absolute zero. At these extreme temperatures, the positron was slowed until it reached its quantum ground state, the lowest possible energy level.
With the positron held in place and stabilized, the team demonstrated the ability to manipulate its quantum states using carefully tuned microwaves. This level of control allows the researchers to measure the positron’s magnetic properties with extreme accuracy. Gabrielse plans to measure the positron’s magnetic moment to a precision of one part in 10 trillion, matching a previous measurement his team achieved with a single electron.
The ultimate goal is a head-to-head comparison between the magnetic moment of the electron and the positron. Current physics theories predict the two values will be identical. However, if the measurements reveal even the slightest discrepancy, it would represent a fundamental break from the Standard Model. According to Gabrielse, finding such a difference would constitute the most significant discovery in physics in the last century, potentially explaining the primary mystery of the universe’s matter-dominated existence.
