Antimatter has the same mass as matter but opposite electromagnetic properties. A positron, the antimatter counterpart of the negatively charged (―) electron, has the same mass as an electron but carries a positive charge (+). When matter and its corresponding antimatter meet, they annihilate each other in pairs while releasing energy. Antimatter is believed to have been generated together with matter at the birth of the universe, but it hardly exists in the surroundings and, even when created, it readily interacts with nearby matter and disappears, making it extremely difficult to control.

A research team led by Gerald Gabrielse, Distinguished Professor in the Department of Physics and Astronomy at Northwestern University in the United States, succeeded in trapping and precisely controlling a single positron for several months in an ultracold vacuum, and published the results on the 21st (local time) in the international academic journal “Physical Review Letters (PRL).”

This achievement is attracting attention because if a positron can be captured and its properties measured with extreme precision to check for any differences from those of the electron, it could help identify the cause of the matter–antimatter asymmetry in the universe, a longstanding problem in physics.

● Experimental basis to compare electrons and positrons

When the universe was first created by the Big Bang, matter and antimatter are thought to have been produced in equal amounts, but in the currently observable universe the amount of matter is overwhelmingly larger. The “Standard Model,” the theory describing the fundamental particles and fundamental forces that make up the universe, does not adequately explain the present matter–antimatter asymmetry.

Professor Gabrielse’s team established an experimental basis to test whether the laws of physics apply identically to matter and antimatter. The team slowed down high-energy positrons emitted during the decay of the radioactive isotope sodium-22 and captured them in a “Penning trap,” which levitates and confines particles in midair using electric and magnetic fields.

The interior of the experimental apparatus was then cooled to 0.3 kelvin (K, unit of absolute temperature), equivalent to minus 272.85 degrees Celsius, and maintained in an ultra-high vacuum. By preventing collisions between the positron and surrounding matter, the team succeeded in stably confining a single positron for several months.

The implemented device was named the “single-positron quantum cyclotron.” Cyclotron motion is the circular motion of a charged particle in a magnetic field. The team also succeeded in inducing and observing transitions of the trapped positron between discrete, stepwise energy levels.

The ultimate goal is to use the developed positron storage technology to measure with extreme precision the “magnetic moment,” the physical quantity that represents the magnitude and direction of the positron’s magnetism, and to compare it with the electron’s magnetic moment.

In previous research, the team measured the electron’s magnetic moment with a precision at the level of about one part in 10 trillion through a quantum cyclotron experiment using a single electron. The latest work is assessed as laying the groundwork for applying a similar level of control technology to positrons.

This study did not discover any new differences in properties between electrons and positrons. The core significance lies in demonstrating technologies for positron trapping, cooling, and quantum state control. If future studies uncover differences in the properties of electrons and positrons that cannot be explained by existing theory, they could provide clues to new physical laws beyond the Standard Model.

● Nobel-winning “neutrino” also offers hints

Other approaches to elucidating matter–antimatter asymmetry are also underway. Scientists are focusing on discovering CP violation in neutrinos, the fundamental particles of the universe, or finding properties that cannot be explained by the Standard Model.

One of the key conditions for explaining the matter–antimatter asymmetry of the universe is “CP symmetry violation (CP violation).” CP violation refers to a phenomenon in which symmetry breaks down when a particle is replaced with its antiparticle (C) and space is inverted as in a mirror (P). CP violation itself was first experimentally demonstrated in 1964 through a particle called the “kaon,” but additional evidence is needed to explain the currently observed “matter dominance” of the universe.

Neutrinos have extremely small mass and no electric charge, so they almost never interact with other matter. A moving neutrino continuously changes among three different types in a process called “oscillation.” Comparing the oscillation patterns of neutrinos and antineutrinos, the antimatter counterparts of neutrinos, makes it possible to test CP violation.

Francis Halzen, a professor at the University of Wisconsin–Madison in the United States, contributed to proposing and constructing IceCube, a massive neutrino observatory embedded in the Antarctic ice, and was awarded this year’s Nobel Prize in Physics. Next-generation neutrino observatories, including China’s “JUNO,” which began operation in August last year, Japan’s “Hyper-Kamiokande,” scheduled to start operating in 2028, and the United States’ “DUNE,” slated to begin operation in the 2030s, are also entering full-scale neutrino exploration.

Efforts are also continuing that assume the existence of particles that have never been observed. Rishav Roshan, a researcher in the School of Physics and Astronomy at the University of Southampton in the United Kingdom, and Sudipta Show, a postdoctoral researcher in the Department of Physics at the Indian Institute of Technology Kanpur, suggested that a theoretically proposed particle called the “heavy neutrino” may have played a crucial role in generating matter–antimatter asymmetry, and published their findings in PRL on the 8th of last month.

Their claim is that heavy neutrinos, assumed to have existed in the early universe, survived for a long time, altered the rate of cosmic expansion, and thereby affected the process by which today’s matter–antimatter asymmetry in the universe was established. The researchers projected that their theory could be tested in the future through observations of traces of primordial gravitational waves or neutrino oscillations generated in the early universe.

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