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Physicists led by Gerald Gabrielse at Northwestern University trapped and studied a single positron for a month using electric and magnetic fields in an ultracold vacuum. The team plans to measure the positron’s magnetic moment and compare it with the electron’s; that measurement has not yet been reported.
Physicists led by Gerald Gabrielse of Northwestern University have trapped and studied a single positron—the antimatter counterpart of an electron—for a month, according to a report in Physical Review Letters. The wall-free electromagnetic trap gives researchers a way to examine the particle’s quantum states over an extended period and is intended to support a precise comparison of positron and electron properties.
The team produced positrons from decaying sodium-22, a radioactive isotope with a half-life of about two and a half years. To slow the particles, researchers sent them through a thin piece of cold tungsten. The positrons lose energy as they pass through the material, though collisions can also cause them to annihilate with matter. Those that emerge are guided onward by electromagnetic fields.
In the final stage, the positron is held in a vacuum at a temperature just 0.3 degrees Celsius above absolute zero. An electric field confines it from above and below, while a magnetic field confines it from the sides. This arrangement has no solid walls for the antimatter to strike. Gabrielse told Scientific American that the team isolated a lone positron for a month and said the method could allow researchers to keep one trapped indefinitely; that longer duration is a possibility, not a reported result.
The next research goal is to measure the positron’s magnetic moment, a property related to the strength and orientation of its magnetic field. The team needs to determine the energy required to flip the particle’s spin and the energy needed to move it from its ground state to its first excited state. The supplied report describes these as future measurements, not completed findings.
A Longer Window for Positron Tests
Holding an individual positron for a month gives physicists time to study it and make repeated measurements without needing to recapture a new particle after a short interval. The key scientific aim is a comparison with the electron’s magnetic moment. Gabrielse’s group has measured the electron’s magnetic moment with a similar setup to a precision of one part in 10 trillion; an equivalent positron measurement could test whether the two values match as expected.
The comparison matters because the Standard Model predicts a specific relationship between matter and antimatter properties. A measured difference could point to physics not captured by that prediction and contribute to research into why the observable universe contains far more matter than antimatter. The trap itself does not explain that imbalance, and the source reports no evidence that the positron and electron differ.
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From Sodium Decay to a Wall-Free Trap
Antimatter is difficult to handle because when it meets ordinary matter, the particles can annihilate. A conventional container would give a positron solid walls to collide with, so this experiment instead uses electromagnetic fields to confine it in a vacuum. The approach depends on slowing the particles first and then steering them into the trapping region.
According to the report, the team’s earlier electron measurement provides a precision benchmark for the planned positron work. More broadly, physicists are testing matter-antimatter symmetry in several ways. The basic motivation is that theories predict a close correspondence between the two, while the early universe appears to have left behind a small excess of matter after most matter and antimatter annihilated. A magnetic-moment comparison is one possible test among many, not a complete account of the cosmic imbalance.
“The big challenge is to persuade these positrons to slow down and then capture them.”
— Gerald Gabrielse, Northwestern University physicist who oversaw the project
The Magnetic-Moment Result Is Pending
The report establishes that the team trapped a single positron for a month, but it does not give a positron magnetic-moment measurement or a comparison with the electron. It therefore remains unknown whether the two magnetic moments match at the precision the researchers seek. No measured difference—and no explanation for the matter-antimatter imbalance—has been reported in the supplied material.
The report also says the method could permit indefinite trapping, but it provides no demonstration of a trap lasting longer than the month described. Details such as a specific calendar date for the journal publication and a timetable for the next measurement are not included in the source material.
The Planned Positron-Electron Comparison
The researchers intend to use the trap to measure the positron’s spin-flip and ground-to-excited-state energies, which are needed to determine its magnetic moment. They can then compare that result with the electron measurement made using a similar setup. The next major development will be the reported measurement and its precision; until those results are available, the trap’s significance lies in enabling the test rather than settling its outcome.
Key Questions
What antimatter particle did the physicists trap?
They trapped a single positron, the antimatter counterpart of an electron. A positron has the same mass as an electron but the opposite electric charge.
How long was the positron held?
The team reported isolating and studying one positron for a month. The report says the method could allow longer trapping, potentially indefinitely, but does not report such a longer duration as achieved.
How can a trap hold antimatter without it touching matter?
The positron was confined in a vacuum using electric fields above and below it and a magnetic field around its sides. The trap has no solid walls for the particle to hit.
Has the experiment shown that positrons and electrons behave differently?
No such difference is reported. The team plans to measure the positron’s magnetic moment and compare it with the electron’s; the result is still pending in the supplied report.
Why does the comparison matter?
The Standard Model predicts a relationship between matter and antimatter properties. A difference in their magnetic moments could provide a clue for researchers studying why the universe contains much more matter than antimatter, but the trap has not yet established such a difference.
Source: hn
