Physicists Kept a Single Antimatter Particle Floating in a Trap for Months

Physicists at Northwestern University in the United States say they have kept a single positron, the antimatter counterpart of the electron, confined in a cryogenic, superconducting vacuum trap for months, according to ScienceAlert. The researchers say the result opens the way to some of the most rigorous tests of particle physics so far.

Positrons are hard to hold on to because they are destroyed the moment they meet an electron, and electrons are nearly everywhere. Here is how the team managed it.

Why is a positron so hard to keep?

Every particle has an antimatter partner, and when a pair meets, both vanish in a small burst of energy. Positrons form naturally in cosmic explosions, in energetic particle interactions and in radioactive decay. Physicists have observed them since the 1930s, but studying one for any length of time requires isolating it from all other matter. An electromagnetic trap can hold a single particle in place in a vacuum, so the harder problem is getting a fast positron into the trap at all.

How did the team slow the positron down?

The researchers used a radioactive isotope of sodium as their source. It emits positrons with energy of up to around 546 kiloelectronvolts, which ScienceAlert described as about 88 percent of the speed of light.

To slow them, the team passed positrons through a single crystal of tungsten. Most of the positrons collide with electrons in the crystal and are wiped out. A few emerge from the surface much slower, loosely paired with an electron in a state called positronium. In the form the researchers used, the electron and positron sit relatively far apart, which delays their annihilation long enough to carry the positron into the trap.

Northwestern physicist Gerald Gabrielse, the paper’s senior author, told ScienceAlert that most positrons from the source are not slowed and captured, and instead end up annihilating with an electron in the apparatus.

How does the trap hold it?

The device is a Penning trap, which uses a combination of electric and magnetic fields to confine a charged particle without letting it touch the walls. Positronium enters through an opening just 640 micrometers wide. Inside, an electric field strips away the electron and leaves the positron behind. The trap is kept in an extremely good vacuum and cooled to 300 millikelvins, a fraction of a degree above absolute zero.

Gabrielse described waiting for a detector signal that shows a captured positron. The team then pulls the radioactive source away so that no further positrons load into the trap.

What did they do with the trapped particle?

Holding the positron was only the first step. Inside the trap’s strong magnetic field, the particle moves in tiny circles, a motion known as cyclotron motion. The researchers cooled it to its lowest quantum energy state and then used carefully tuned microwaves to nudge it between discrete energy levels, one quantum at a time. According to ScienceAlert, they could also detect those jumps, along with flips in the particle’s spin, without disturbing the quantum states being measured.

Gabrielse said watching a positron make one-quantum energy jumps was “extremely satisfying” after years of pursuing the goal. He credited physicist Tom Myers, who led the team’s solution and whose doctoral thesis work culminated in the result.

Why it matters: what could this be used for?

The work belongs to a long-running effort to explain a puzzle. Theory says the birth of the Universe should have produced equal amounts of matter and antimatter, yet the present Universe is heavily skewed toward matter. Antiparticles’ habit of colliding with ordinary particles makes that mystery harder to study.

Being able to hold and control a single positron for months is meant to allow far more precise measurements. ScienceAlert pointed to a fundamental principle called CPT symmetry, short for charge, parity and time reversal, as one of the main reasons to trap and measure a positron. For general readers, the practical point is that this is basic research: it improves the tools physicists use to test their most basic theories, rather than leading to a product.

Around Prague, based on reporting by ScienceAlert

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