Antimatter is exceptionally difficult to study because it annihilates when it comes into contact with ordinary matter. A team of physicists has now trapped a single positron—the antiparticle of the Electron—and kept it suspended for months, creating new possibilities for precision experiments.
The achievement, published in Physical Review Letters, relies on what the researchers call a “single-positron quantum cyclotron.” The device is designed to isolate the particle sufficiently to examine one of the central assumptions of modern physics: that electrons and positrons should have identical masses and magnetic moments, with opposite signs.
The positrons are produced by a radioactive sodium-22 source and must be cooled to around 300 millikelvin, just a fraction of a degree above absolute zero. According to the researchers, this reduces their energy by more than ten orders of magnitude over a very short period.
To load the particle into the trap, the positron briefly combines with an electron to form positronium, an electrically neutral exotic atom. That atom can pass through a tiny 625-micrometer opening into a trapping region, where an intense electric field ionizes it and leaves the positron confined.
The procedure is highly demanding: loading a single positron takes between eight and 11 hours. Once captured, however, the particle can remain isolated for months, allowing scientists to study it under controlled conditions.
The motivation behind the work is linked to one of physics’ biggest unanswered questions: why the universe contains far more matter than antimatter. The Standard Model suggests that the Big Bang should have produced equal quantities of both. A difference between electrons and positrons could therefore point to a violation of the expected symmetry.
In 2023, the same Northwestern University team measured the electron’s magnetic moment, expressed through its g-factor, with a precision of 0.13 parts per billion. The existing direct comparison between the electron and positron dates from 1987 and found them equal to within 2.1 parts per billion.
The new cyclotron is intended to improve the precision of the positron’s magnetic-moment measurement by approximately 30 times. The apparatus has now demonstrated that it can capture and hold the particle, but the high-precision measurement itself has not yet been completed.
Research leader Gerald Gabrielse said that if electrons and positrons were found not to be identical, it would represent “the biggest discovery in physics of the last century.”
Sources: Xataka.