One positron, held for months: Northwestern physicists trap a single antimatter particle

What happened

A team of physicists trapped a single antimatter particle for months. ^[1] The work was done by researchers at Northwestern University, and the findings were published in Physical Review Letters. ^[2][1] Gerald Gabrielse is the lead researcher on the experiment. ^[1]

The particle is a positron, the antimatter counterpart of the electron. ^[2] The two have exactly the same mass, but their electric charges are opposite. ^[2]

Why antimatter is hard to hold

Antimatter is hard to study because it is destroyed on contact with ordinary matter. ^[3] It explodes when it meets ordinary matter. ^[1] Any container with walls would therefore end the experiment.

The team avoided walls entirely. ^[1] They suspended the positron in a Penning trap, which uses electric and magnetic fields to hold a particle in the middle of an empty, nearly perfect vacuum. ^[3] The device is called a one-positron quantum cyclotron, and it keeps the particle floating without touching anything. ^[1][2]

Catching and cooling the particle

The source of the positrons was sodium-22, a radioactive isotope whose decay emits positrons with great energy. ^[3] The researchers first caught a particle as it shot out of the radioactive material. ^[1] Slowing it enough to catch it was the hardest part of the experiment. ^[1]

Once caught, the particle was cooled to 0.3 degrees above absolute zero. ^[1] The trap was chilled to about the same temperature. ^[3] Loading a positron takes 8 to 11 hours with the team's simple, low-intensity source. ^[3]

Controlling the quantum states

The cold lets the team control the particle's quantum states precisely without touching it. ^[2] The experiment put the positron into its lowest possible energy state. ^[1] That allowed scientists to control it for months without destroying it. ^[1]

The team used tuned microwaves to move the positron up its energy ladder and flip its spin. ^[3] The size of the frequency shifts indicated that only one positron was in the trap. ^[3] Physicists manipulated its quantum states one jump at a time. ^[3]

Keeping the particle for months lets the researchers repeat measurements. ^[2] Physicists have made very precise measurements with electrons, but reaching the same precision with positrons has proven more difficult. ^[2]

What a mismatch would mean

Mathematics predicts that every particle of matter has an equivalent antiparticle, and scientists have verified this experimentally. ^[2] The Standard Model predicts that electrons and positrons should be perfect mirror images, a principle known as CPT symmetry. ^[3] A mismatch between the magnetic moments of the electron and positron would show that the theory is incomplete. ^[3]

The study aims to learn whether antimatter behaves differently than the matter it mirrors. ^[1] Almost everything observed is made of ordinary matter, and the reason is still unknown. ^[2] Under the Big Bang theory, matter and antimatter should have wiped each other out billions of years ago. ^[1] The Big Bang should have created equal amounts of both, yet nearly everything is regular matter, and this result could help address that. ^[1]

Scientists have increasingly doubted the Big Bang theory and have theorized that the universe may have been created by a bounce inside a black hole. ^[1]