An artist's conception of an antihydrogen atom trapped in the eight-pole… (Katie Bertsche / Lawrence…)
Researchers at the Large Hadron Collider near Geneva have trapped atoms of the elusive antimatter form of hydrogen for nearly 17 minutes, a major step toward understanding what happened to this mysterious substance when the universe was created 13.6 billion years ago.
Physicists plan to study the antihydrogen intensively to see how it interacts with gravity and other forces of nature, looking for slight differences between its behavior and that of normal hydrogen. Such differences might explain why normal matter dominates the universe and antimatter is virtually nonexistent.
Current theories hold that matter and antimatter were created in equal quantities during the Big Bang. Antimatter could have been annihilated when it came into contact with normal matter, but if they were formed in equal quantities, there would be no universe left. Researchers hope that examining the physical properties of antihydrogen and other forms of antimatter in precise detail will explain this discrepancy.
But first they have to get enough of the material and hold it in one place long enough to study — and that is the significance of the new experiments reported Sunday in the journal Nature Physics.
"We have never talked about holding on to these things for so long" in the past, said physicist Jeffrey Hangst of Aarhus University in Denmark, spokesman for the group reporting the achievement. "If you want to study these antiatoms, you need to use electromagnetic radiation, microwaves, lasers and other tools," and that means the antiatoms must be confined for at least several minutes.
In November, the same team reported trapping antihydrogen for as long as 0.2 of a second. Although that report demonstrated that antiatoms could be contained, the time was not long enough for individual atoms to settle down into their so-called ground state, in which positrons orbiting the antiproton nucleus reach their lowest, least energetic orbits.
Accurate results can be obtained only when measurements are made in the ground state. The new experiment demonstrates that the team can confine the antiatoms long enough for them to relax sufficiently to be measured.
The key to the new work is a trap using an eight-magnetic-pole series of magnets to confine the antiatoms, preventing them from annihilating themselves on the walls of the container. Because antihydrogen, like hydrogen, is electrically neutral, such containment is difficult. But because each antiatom acts like a very weak magnet, the feat can be achieved if the antiatoms are moving slowly enough — that is, if they have an energy corresponding to a temperature less than half a degree above absolute zero.
The researchers achieve that by passing antiprotons produced in the accelerator through an electron cloud and filters that slow it down before it enters the trap. In the trap, the antiproton is combined with a positron produced by radioactive disintegration to produce slowly moving antihydrogen atoms, which can be trapped.
But to study the antiatoms' interaction with gravity, Hangst said, they need to be cooled to a temperature less than a thousandth of a degree above absolute zero. That can be achieved with lasers, but the current magnetic containment chamber does not allow laser access. Researchers are building a new one that does, which should be available next year.
Meanwhile, researchers have begun studying the antiatoms with microwaves and electromagnetic fields, searching for their first clues to how the universe really began.