Single Positron Trapped in a Quantum Cyclotron to Test CPT Symmetry
Physicists at Northwestern University have held a single positron, the antimatter twin of the electron, in a "quantum cyclotron", opening a new way to test a key prediction of the Standard Model of particle physics. The work, led by Gerald Gabrielse, was published online on October 21 in Physical Review Letters.
The positron has the same mass as the electron but an equal and opposite electric charge. Like the electron, it also behaves like a tiny bar magnet, and according to the Standard Model its magnetic moment, the strength of that magnet, should be exactly equal and opposite to the electron's.
Why compare a positron with an electron
The prediction stems from CPT symmetry. It states that if every particle were swapped for its antiparticle (C), the universe flipped into its mirror image (P) and time run backward (T), all at once, the laws of physics would be unchanged. One of the most sensitive tests is to compare the positron's magnetic moment with the electron's. Any difference would hint at new physics beyond the Standard Model.
Electron measurements have reached an accuracy of around 1 part in 10 trillion, while 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 for far more precise measurements.
How the positron was caught
The team took positrons from a radioactive sodium source, which flings them out at extremely high speeds, and slowed them down dramatically. To confine a single positron, they first let it pair with an electron into positronium, an electrically neutral atom. The positronium was torn apart inside the trap, which left the positron behind.
The antiparticle was then 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 cooled to just a fraction of a degree above absolute zero, so the positron could settle into its lowest-energy circling motion.
Reading out motion and spin
With the positron's energy divided into distinct, clearly separated steps, the researchers 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. They read out these states by monitoring tiny shifts in the positron's back-and-forth bobbing within the trap, which lets them detect both its motion and spin without disturbing either.
Once captured, the positron can be stored for several months, according to the report. This paves the way for future experiments that could measure its magnetic moment with an accuracy rivaling existing electron measurements. The magnetic moment itself has not been measured in this work: the paper demonstrates the trapping and the quantum-cyclotron method.
The work is part of the science coverage on akm.news.