IN AND OUT: This artist's conception depicts the confined path of an antimatter atom inside the ALPHA trap in light blue. The white tracks depict outflying particles that originate from a matter-antimatter annihilation when the antiatom is released from its trap. Image: ALPHA/CERN
Maybe antimatter is finally ready for its close-up. A team of physicists has succeeded in producing rudimentary atoms of antimatter and holding on to them for several minutes, an advance that holds hope for detailed comparisons of how ordinary atoms of matter compare with their exotic antimatter counterparts.
The researchers, from the ALPHA antimatter experiment at CERN, the European laboratory for particle physics, reported last year the first trapping of antihydrogen, the simplest antimatter atom. But the antihydrogen had at that time been confined for less than two tenths of a second. That interval has now been extended by a factor of more than 5,000. In a study published online June 5 in Nature Physics, the ALPHA group reports having confined antihydrogen for 16 minutes and 40 seconds. The more relevant number for physicists, who often deal in powers of 10, is 1,000 seconds. (Scientific American is part of Nature Publishing Group.)
The subatomic particles of everyday matter—protons, neutrons and electrons—have antimatter cousins; when matter meets antimatter the two annihilate in a burst of energy. And just as the neutral hydrogen atom is made of a single proton bound to an electron, an atom of antihydrogen comprises an antiproton and a positron, the antimatter counterparts, respectively.
But the mutual annihilation between those particles and their ubiquitous matter counterparts makes it challenging to hang on to antimatter for very long, and even more challenging to produce and confine bound atomic arrangements of multiple antiparticles. Neutral anti-atoms such as antihydrogen are especially tricky to confine because they are impervious to electric fields, which can be used to steer charged antiparticles such as antiprotons. Experiments such as ALPHA instead use superconducting magnets to trap their quarry.
Cagey as anti-atoms are, physicists would like to pin them down and compare the properties of antihydrogen with hydrogen, the most abundant element in the universe. Those comparisons might involve laser spectroscopy of the anti-atoms or physical tests of how antihydrogen "feels" the influence of gravity. Any discrepancies between hydrogen and antihydrogen might help explain why matter won out over antimatter in our observable universe. "If one could do that, that would be a huge advance in terms of understanding why we live in a world of matter," says Clifford Surko, a physicist at the University of California, San Diego, who wrote a commentary for Nature Physics accompanying the new study. "There's got to be an asymmetry somewhere, so that's a long-term goal."
The lifetime of antihydrogen in the ALPHA trap is probably sufficient to begin those studies. "We think we're in a position to start measuring something," says ALPHA spokesperson Jeffrey Hangst of Aarhus University in Denmark. Initial studies will involve irradiating the anti-atoms with microwaves to try to engage them in a resonant interaction, flipping their spin like a compass needle swinging from north to south.
Critically, the confinement times achieved by ALPHA imply that the antihydrogen atoms have had time to decay into their lowest-energy, or ground, state. "This method of antihydrogen formation creates them in highly excited states," Surko says. "They're fragile, and for really high-precision measurements of antihydrogen you need them in the ground state."
Hangst says that the jump from trapping times measured in milliseconds to those measured in hundreds of seconds did not stem from any one major advance. But getting antihydrogen atoms to stick in the trap much more often was a big help in improving on last year's confinement experiment. It now takes fewer experimental runs to demonstrate that an antihydrogen atom has been trapped. "What was tricky here was not keeping them but trapping enough of them to do the experiment," he says. "The big technological step here is we're much better now at trapping them at all."
Still the efficiency of trapping is somewhat low—for each antiatom confined by the trap, thousands more from the same batch escape. And in 16 trapping experiments of 1,000 seconds each, only seven antihydrogen atoms were detected in total. (The researchers demonstrate the confinement of antihydrogen by quickly shutting down the superconducting magnets, turning the anti-atoms loose, and watching for matter–antimatter annihilations on the walls of the trap.)
A competing antimatter experiment at CERN, known as ATRAP, has been working toward producing larger numbers of antihydrogen atoms with lower kinetic energies, which would facilitate their trapping. But so far that effort has yet to bear fruit. "We think that it would be good to have more atoms than [the ALPHA rate of] fewer than one atom per trial," says Harvard University physicist Gerald Gabrielse, spokesperson for the ATRAP collaboration. "We would hope not to be publishing a paper that says we see 0.6 atom per trial, but 100 atoms per trial."
The ATRAP group, Gabrielse says, made the choice to increase the number of atoms in the trap rather than increasing the sensitivity of the instruments to detect small numbers of anti-atoms, as ALPHA has done. "Maybe we made the wrong one," he says of that decision. "Certainly we made the one that got less publicity." Nevertheless, Gabrielse says he is encouraged by his competitors' success. "I'm glad that they have demonstrated that you can trap antihydrogen atoms," he says. "I think it shows that if we have more atoms, we'll have time to do some things with them."