An isolated positron prepares a closer matter–antimatter comparison
· BehindLines newsroom
A team at Northwestern University has kept a positron in an electromagnetic trap for several months and distinguished its quantum states. Published on 21 October in Physical Review Letters, the experiment prepares the way for a more precise comparison with the electron. It does not yet report a difference between matter and antimatter.
Holding a particle without walls
The positron is the electron’s antiparticle. When they meet, they annihilate, so an ordinary container cannot hold it. The team led by Gerald Gabrielse uses a Penning trap, whose electric and magnetic fields keep the particle away from the walls.
The positrons come from the decay of sodium-22. The researchers first have to slow them down and then capture one in an almost empty environment. The apparatus is cooled to about 300 millikelvins, or 0.3 degrees above absolute zero. The small number of possible collisions helps extend confinement.
From confinement to measurement
The result goes beyond storage. Microwaves can induce transitions between energy levels, which the apparatus can detect. This control turns the positron’s motion into a quantum measurement tool.
The next objective concerns its magnetic moment, a property describing its behaviour in a magnetic field. The authors want to compare it with the electron’s to test CPT symmetry, a fundamental principle of the standard model of particle physics. That comparison remains an objective: this work establishes no violation of the symmetry.
A recent publication of previously shared work
21 October is the date of the journal publication and the university announcement. The manuscript was already available on arXiv on 3 May, with a revision on 21 August. Publication therefore does not mean that the experiment took place this week.
The advance mainly provides a new method for examining a particle that is difficult to retain. Explaining why matter predominates in the Universe would require other results. The trap makes tests possible without already supplying their answers.