Physicists hold single antimatter particle for months

Physicists at Northwestern University have caught a single positron, the antimatter counterpart of the electron, and cooled it to its quantum ground state inside a Penning trap. The team calls the device a "one-positron quantum cyclotron," and the particle has stayed confined for months. The study, led by Gerald Gabrielse, was reported on Wednesday, October 21, in Physical Review Letters. The researchers want to use the device to measure the positron's magnetic moment and check it against the electron's. That comparison would test a basic symmetry of the Standard Model.phys.org+2

Catching antimatter without walls

The positrons come from radioactive sodium-22, which emits them with energies of up to 546 kilo-electron-volts. The trap is held at 300 millikelvin, which corresponds to a thermal energy more than 10 orders of magnitude lower. The positrons are first slowed in a tungsten moderator. Some then pick up an electron and form electrically neutral positronium, which can pass through a 625-micrometer opening into the trap's nearly closed microwave cavity. Inside, the trap's electric field pulls the electron away and the positron stays behind. Loading a single positron currently takes 8 to 11 hours.physics.aps.org

"If you want to hold on to antimatter, you have to hold it in a container without walls," Gabrielse told Scientific American. The team also showed it could add precise amounts of energy with tuned microwaves and detect the positron's individual quantum jumps. Thomas Myers, a doctoral student, is the paper's first author.scientificamerican.com+1

A mirror test for the Standard Model

Gabrielse's group has already used a one-electron quantum cyclotron to measure the electron's magnetic moment to about one part in 10 trillion. The positron measurement has fallen behind. The best electron-positron comparison still comes from a 1987 experiment, which found the two g-factors equal to within 2.1 parts per trillion. ETH Zurich physicist Paolo Crivelli wrote that the new device opens a path to a 30-fold gain in precision for the positron.phys.org+1

Under CPT symmetry, a particle and its antiparticle should have magnetic moments that are equal in size and opposite in sign. A confirmed difference "would challenge the very foundations" of relativistic quantum field theory, Crivelli wrote. It could also help explain why the universe contains far more matter than antimatter. Fermilab theorist Roni Harnik, who was not involved in the work, called it "an exciting development that will enable us to test a basic symmetry".physics.aps.org+1

"Current theories suggest that there will be no difference in the two measurements," Gabrielse said. "If it turns out they are not equal, that would be the biggest discovery in physics in the last century".phys.org