the antimatter counterpart of an electron - into an exquisitely controlled quantum state.
After capturing the fast-moving particle from radioactive decay, the scientists cooled it to nearly absolute zero. Then, they suspended the particle in a near-perfect vacuum, preventing it from touching ordinary matter - an encounter that would cause it to vanish.
By trapping and cooling a positron this way, the scientists created what they call a "one-positron quantum cyclotron." In this state, the positron essentially behaves much like an artificial atom with quantum energy levels scientists can precisely control and observe.
The advance ultimately could enable the most precise test yet of a fundamental symmetry between electrons and their antimatter counterparts. According to the Standard Model of particle physics, electrons and positrons should be nearly perfect mirror images of one another. By measuring the positron's magnetic properties with unprecedented precision and comparing those to the electron's properties, physicists could put this fundamental symmetry to its most stringent test yet. If the measurements don't match as expected, they would reveal a crack in the Standard Model, offering new clues to enduring mysteries, including why ordinary matter, rather than antimatter, dominates the universe.
The study was published today (Oct. 21) in Physical Review Letters.
"Antimatter gets its popular charm from science fiction," said Northwestern's Gerald Gabrielse, who led the study. "If you take an electron and an anti-electron (or positron) and collide them together, they both disappear. We describe that as annihilation, a concept that fuels science fiction writers' imaginations. The big mystery of the universe is the Big Bang should have created equal amounts of matter and antimatter. But, if that were true, it would annihilate, and we'd be left with nothing. There's clearly something wrong with that theory, and that's what we're trying to figure out."
Gabrielse is the Board of Trustees Professor of Physics at Northwestern's Weinberg College of Arts and Sciences, where he directs the Center for Fundamental Physics. Thomas Myers, a Ph.D. student in Gabrielse's laboratory, is the paper's first author.
Looking for cracks in physics
A longstanding theory, the Standard Model describes the fundamental particles and three of the four fundamental forces (electromagnetic, strong nuclear and weak nuclear forces) in the universe. Experiment after experiment has confirmed the model's predictions with extraordinary precision. Yet physicists know the model is incomplete. Among its shortcomings, it cannot explain why the observable universe contains overwhelmingly more matter than antimatter.
"We know the Standard Model has to be wrong somewhere - or at least incomplete," Gabrielse said. "For example, it cannot predict why the universe exists, so that's a pretty big loophole. We're very precisely examining the most fundamental and simple systems to find where the theory breaks down. We hope to find an unambiguous answer."
Gabrielse has spent his decades-long career searching for that answer. His team previously developed techniques to isolate a single electron and measure its magnetic properties with extraordinary precision. He also pioneered methods to slow, capture and cool antiprotons, helping launch the field of low-energy antimatter research. Now, the new work brings those two lines of research together: applying the quantum control achieved with electrons to a single particle of antimatter.
A container with no walls
Nearly everything on Earth is made from just three familiar particles: electrons, protons and neutrons. An antimatter universe, instead, could be built from their antimatter counterparts: positrons, antiprotons and antineutrons. In fact, if everything on Earth were antimatter, Gabrielse says it would be extraordinarily difficult to tell the difference.
The fireworks occur when the two worlds meet. When an electron encounters a positron, the pair annihilates. That means scientists cannot simply place a positron in a conventional container. Touching the container itself would most certainly cause the particle's demise.
"If we want to study antimatter, we must handle it carefully," Gabrielse said. "To be clear, it's not dangerous. We don't have enough antimatter to make an explosion."
To carefully handle the particle, the Northwestern team used a state-of-the-art trap, which combines specialized electric and magnetic fields to suspend the positron in an empty space. Sitting in the middle of the container, the positron never touches anything. Because of this setup, the positron in Gabrielse's lab has remained trapped for months.
From speeding to stillness
Catching the positron in the first place was its own challenge. Positrons can be produced during the radioactive decay of certain atomic nuclei. In this experiment, Gabrielse used nuclei from sodium-22, a rare, radioactive isotope of sodium. When the particles emerge from the nuclei, they carry enormous amounts of energy.
The team first slowed down the positron dramatically and then guided it into the trap's near-perfect vacuum. The interior of the trap is cooled to about 300 millikelvins, or just 0.3 degrees above absolute zero.
"We cool it down so much that it's in its quantum ground state," Gabrielse said. "We actually coaxed this very fast positron to sit there and behave for months at a time."
The ultimate test
Gabrielse and his team also demonstrated that they could precisely manipulate and detect the positron's individual quantum energy states. By delivering carefully tuned microwaves, the team added tiny precise amounts of energy to the positron and detect its resulting quantum transitions. That level of control is essential for the scientists' ultimate goal: measuring the positron's magnetic properties with extraordinary precision and comparing them with those of an electron.
In previous work, Gabrielse and his team used a one-electron quantum cyclotron to measure the electron's magnetic moment to about one part in 10 trillion, which is among the most precise measurements ever made of an elementary particle. With the trapped antimatter, Gabrielse now plans to make the same kind of quantum measurement with a positron - then will put the two results head to head.
"Current theories suggest that there will be no difference in the two measurements," Gabrielse said. "If it turns out they are not equal, that would be the biggest discovery in physics in the last century."
The study, "One-positron quantum cyclotron," was supported by the National Science Foundation (grant number 2110565).
