Physicists trap a single antimatter particle for months to understand how matter began to dominate the universe after big bang

Physicists at Northwestern University have managed to capture and control a single particle of antimatter for months, creating a new way to investigate one of the biggest unanswered questions in science: why does the universe contain so much more matter than antimatter?

The researchers trapped a positron, the antimatter counterpart of an electron, in an electromagnetic field and cooled it to an extremely low temperature. The achievement, described in a study published in Physical Review Letters, gives scientists greater control over the particle's quantum states and brings them closer to testing whether matter and antimatter behave exactly as existing physics predicts.

The experiment does not yet establish a difference between matter and antimatter. Instead, it creates a controlled environment in which researchers can look for one with extraordinary precision.

How scientists trapped antimatter without letting it disappear

Antimatter is difficult to study because it annihilates when it encounters its corresponding matter particle. When an electron meets a positron, both particles disappear in an annihilation event, releasing energy.

That makes a conventional container unsuitable for holding a positron. If the particle touches the container's walls, it can annihilate on contact.

To avoid this, the Northwestern team used a specialised trap that combines electric and magnetic fields to suspend the positron in a near-perfect vacuum. The particle remains isolated from ordinary matter rather than being held inside a physical container in the usual sense.

According to the researchers, the positron in their laboratory has remained trapped for months.

The team produced the particle using sodium-22, a radioactive isotope of sodium. Positrons released during radioactive decay initially move at high speeds and carry substantial energy. Scientists first had to slow the particle down before guiding it into the trap.

The trapping apparatus was cooled to about 300 millikelvins, equivalent to 0.3 kelvin, or approximately 0.3 degrees above absolute zero. This extreme cooling helped the researchers bring the positron into its quantum ground state, the lowest available energy state in the system.

Gerald Gabrielse, the Northwestern physicist who led the research, said the team had coaxed a fast-moving positron into a state in which it could remain controlled for extended periods.

What is a one-positron quantum cyclotron?

The researchers call their setup a "one-positron quantum cyclotron". In this arrangement, the positron behaves like a highly controlled quantum system with discrete energy levels that scientists can manipulate and observe.

The team demonstrated that it could use carefully tuned microwaves to deliver precise amounts of energy to the particle. By detecting the resulting transitions between quantum energy states, the researchers showed that they could control and measure the positron's behaviour.

This matters because precision measurements require scientists to know exactly how a particle responds to carefully controlled changes in energy.

The experiment brings together two areas of Gabrielse's earlier research: techniques for measuring the properties of individual electrons and methods for slowing, capturing and cooling antimatter particles.

His team previously developed a one-electron quantum cyclotron to measure an electron's magnetic moment with extraordinary precision. The new experiment applies similar quantum control techniques to a positron.

The next step is to compare an electron with its antimatter counterpart

The team's ultimate goal is to measure the positron's magnetic properties and compare them directly with those of an electron.

According to the Standard Model of particle physics, electrons and positrons should have corresponding properties consistent with a fundamental symmetry between matter and antimatter. Their electric charges are opposite, while their other relevant properties are expected to follow precise relationships.

Gabrielse's earlier electron measurements reached a precision of about one part in 10 trillion, among the most precise measurements ever made of an elementary particle.

The researchers now want to carry out a comparable measurement with the trapped positron. The new study establishes the control needed for that work, although it does not yet report the final electron-positron comparison.

"If it turns out they are not equal, that would be the biggest discovery in physics in the last century," Gabrielse said.

Any discrepancy would need to be examined carefully, but a confirmed mismatch could point towards physics beyond the Standard Model.

Why a tiny particle could help explain the universe

The Standard Model describes fundamental particles and three of the four known fundamental forces: electromagnetic, strong nuclear and weak nuclear forces. Its predictions have survived numerous experiments, often with remarkable accuracy.

However, it does not explain everything. One of its biggest unresolved problems concerns the imbalance between matter and antimatter in the observable universe.

According to current cosmological understanding, the Big Bang should have produced matter and antimatter in nearly equal amounts. If the two had remained perfectly balanced and behaved symmetrically, annihilation would have removed much of both, leaving little ordinary matter from which stars, planets and life could form.

Yet the universe we observe is dominated by matter.

Scientists have spent decades investigating whether subtle differences between matter and antimatter could help explain this imbalance. A precise comparison of electron and positron properties offers one way to test the symmetries that underpin current theories.

The new experiment does not solve the matter-antimatter mystery, and the researchers have not demonstrated a failure of the Standard Model. Its significance lies in creating a more controlled method for investigating a fundamental question.

Gabrielse said his team is examining simple physical systems with extreme precision in the hope of finding where current theory may be incomplete.