This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The positron is the antimatter counterpart of the electron, identical in mass but carrying an equal and opposite electric charge. Like the electron, it also behaves like a tiny bar magnet, and according to a key prediction of the Standard Model of particle physics, its magnetic moment should be exactly equal and opposite to the electron's.
Through new research published in Physical Review Letters, Gerald Gabrielse and colleagues at Northwestern University in Illinois have unveiled a promising new way to test this assumption, by trapping a single positron in a "quantum cyclotron." The prediction stems from a principle called CPT symmetry. It states that if we swapped every particle for its antiparticle (C), flipped the universe into its mirror image (P), and ran time backward (T), all at once, the laws of physics would be unchanged. In the process, observers like us likely wouldn't even notice.
One of the most sensitive tests of this principle would be to compare the positron's magnetic moment with that of an electron. If any difference emerged between the two values, it would hint at new physics beyond the Standard Model. However, while electron measurements have reached an accuracy of around 1 part in 10 trillion, the best positron comparison is several orders of magnitude less precise.
The main obstacle is getting a positron into an environment isolated and cold enough to make far more precise measurements possible. In their study, Gabrielse's team tackled this challenge by taking positrons from a radioactive sodium source, which flings them out at extremely high speeds, before slowing them down dramatically. To confine a single positron, they first let it pair up with an electron to form an electrically neutral atom called positronium, which was torn apart inside the trap, leaving the positron behind.
Here, the antiparticle was held in a "Penning trap," which uses electric and magnetic fields to keep the particle circling on the spot—much like a miniature version of the cyclotrons used in particle accelerators. The trap was then cooled to just a fraction of a degree above absolute zero, allowing the positron to settle into its lowest-energy circling motion. With the positron's energy now divided into distinct, clearly separated steps, the team could nudge it up and down one step at a time using microwaves and track flips in its spin: the property linked to its magnetism.
Crucially, they read out these states by monitoring tiny shifts in the positron's back-and-forth bobbing within the trap, allowing them to detect both its motion and spin without disturbing either. Once captured, the positron can be stored for several months. This paves the way for future experiments that could measure its magnetic moment with an accuracy rivaling existing electron measurements—potentially around 30 times better than before.
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