Antimatter is something of science fiction: when a particle meets its antiparticle, both can annihilate and disappear as such, releasing energy. But for physicists, the real mystery is not that encounter, but a much deeper question: If the universe was born in the Big Bang, why is it composed mainly of matter and not reduced to energy after the annihilation of equivalent amounts of matter and antimatter?
To look for clues about this asymmetry, a team from Northwestern University, in the United States, has managed to trap a single positron (the antiparticle of the electron), cool it to almost absolute zero and control its quantum states. The result, published in Physical Review Lettersconstitutes a step towards a precision comparison between the magnetic properties of the electron and those of its antimatter counterpart.
The leader of the study is Gerald Gabrielse, who has spent decades developing techniques to study elementary particles with extraordinary precision. Your team has managed to the positron remains confined for months in a trap that prevents it from coming into contact with ordinary matter. It is not a conventional container: the particle is suspended in a vacuum by electric and magnetic fields, without touching walls or surfaces that would cause its annihilation.
“If we want to study antimatter, we must handle it carefully”explains Gabrielse. Although the word may evoke explosions, the scientist emphasizes that the amount used in the experiment is too small to represent that danger. The challenge is not to contain a large reserve of energy, but to keep a single particle isolated and master its behavior.
The positron does not arrive at the laboratory willing to sit still either. Gabrielse obtained it from the radioactive decay of sodium-22, an unstable isotope that emits these antiparticles with high energy. First it was necessary to stop him and drive him to the trap. The environment was then cooled to about 300 millikelvin, just 0.3 degrees above absolute zero.the limiting temperature at which thermal movement reaches its minimum.
Under these conditions, Gabrielse’s team managed to bring the positron to its fundamental quantum state, the lowest energy level it can occupy in the system. Physicists describe the device as a “quantum positron cyclotron”: a system in which the particle behaves, in some respects, like an artificial atom, with defined energy levels that can be manipulated and measured.
To do so, Gabrielse’s team applied precisely tuned microwaves. By supplying tiny amounts of energy, they were able to trigger transitions between quantum states and detect the particle’s response. This control is essential for the final objective: measure with enormous precision the magnetic moment of the positron and compare it with that of the electron. What allows us to know the magnetic moment is how a particle responds to a magnetic field and constitutes one of its fundamental properties. The comparison has a special value because The Standard Model of particle physics predicts that the electron and positron should behave as mirror images: They have the same mass and opposite electrical charges, and their magnetic properties must have an extremely precise relationship.
Gabrielse’s team had previously used a similar trap to measure the electron’s magnetic moment with an accuracy of about one part in ten trillion, one of the most precise measurements ever made on an elementary particle. Now he intends apply a technique equivalent to the positron and confront both results. The novelty, therefore, is not that a difference between matter and antimatter has been demonstrated, but that a tool has been prepared to search for it with exceptional precision.
Why does a potential discrepancy matter? The Standard Model has passed countless experimental tests, but it does not offer a complete explanation of nature. Among its great open questions is the predominance of matter in the observable universe. If the properties of the electron and positron didn’t match what theory predicts, physicists would have a sign that something fundamental needs to be revised. That wouldn’t automatically explain the cosmic imbalance, but it would provide an unexpected clue.
For now, symmetry remains the theoretical prediction. The experiment has not found a crack in the known laws: it has managed to keep an antimatter particle trapped for months and its quantum states can be controlled and detected. The next step will be to measure its magnetism with the necessary precision to compare it with that of the electron.
“If it turns out that they are not the same, it would be the greatest discovery in physics in the last century – adds Gabrielse -. We know that the Standard Model has to be wrong in some aspect or at least that it is incomplete. For example, it cannot explain why the universe exists; this is, therefore, a rather important gap. We are examining with great precision the most fundamental and simple systems to discover where the theory fails. We hope to find an unequivocal answer”.