SCIENCE

To join the positrons in their central well, the antiprotons must be carefully nudged by an oscillating electric field, which increases their velocity in a controlled way through a phenomenon called autoresonance.

“It’s like pushing a kid on a playground swing,” says Fajans, who credits his former graduate student Erik Gilson and Lazar Friedland, a professor at Hebrew University and visitor at Berkeley, with early development of the technique. “How high the swing goes doesn’t have as much to do with how hard you push or how heavy the kid is or how the long the chains are, but instead with the timing of your pushes.”

Antiprotons and positrons are brought into the ALPHA trap from opposite ends and held there by electric and magnetic fields. Brought together, they form anti-atoms neutral in charge but with a magnetic moment. If their energy is low enough they can be held by the octupole and mirror fields of the Minimum Magnetic Field Trap. (Click on image for best resolution.)

The novel autoresonance technique turned out to be essential for adding energy to antiprotons precisely, in order to form relatively low energy anti-atoms. The newly formed anti-atoms are neutral in charge, but because of their spin and the distribution of the opposite charges of their components, they have a magnetic moment; provided their energy is low enough, they can be captured in the octupole magnetic field and mirror fields of the Minimum Magnetic Field Trap.

Of the thousands of antihydrogen atoms made in each one-second mixing session, most are too energetic to be held and annihilate themselves against the trap walls.

Setting the ALPHA 38 free

After mixing and trapping—plus the “clearing” of the many bare antiprotons that have not formed antihydrogen—the superconducting magnet that produces the confining field is abruptly turned off—within a mere nine-thousandths of a second. This causes the magnet to “quench,” a quick return to normal conductivity that results in fast heating and stress.

“Millisecond quenches are almost unheard of,” Fajans says. “Deliberately turning off a superconducting magnet is usually done thousands of times more slowly, and not with a quench. We did a lot of experiments at Berkeley Lab to make sure the ALPHA magnet could survive multiple rapid quenches.”

From the start of the quench the researchers allowed 30-thousandths of a second for any trapped antihydrogen to escape the trap, as well as any bare antiprotons that might still be in the trap. Cosmic rays might also wander through the experiment during this interval. By using electric fields to sweep the trap of charged particles or steer them to one end of the detectors or the other, and by comparing the real data with computer simulations of candidate antihydrogen annihilations and look-alike events, the researchers were able to unambiguously identify 38 antihydrogen atoms that had survived in the trap for at least 172 milliseconds—almost two-tenths of a second.

Says Fajans, “Our report in Nature describes ALPHA’s first successes at trapping antihydrogen atoms, but we’re constantly improving the number and length of time we can hold onto them. We’re getting close to the point where we can do some classes of experiments on antimatter atoms. The first attempts will be crude, but no one has ever done anything like them before.”

“Trapped Antihydrogen,” by Gorm Andresen, Mohammad Dehghani Ashkezari, Marcelo Baquero-Ruiz, Will Bertsche, Paul Bowe, Eoin Butler, Claudio Lenz Cesar, Steve Chapman, Michael Charlton, Adam Deller, Stefan Eriksson, Joel Fajans, Tim Friesen, Makoto Fujiwara, Dave Gill, Andrea Gutierrez, Jeffrey Hangst, Walter Hardy, Mike Hayden, Andrew Humphries, Richard Hydomako, Matthew Jenkins, Svante Jonsell, Lars Jørgensen, Leonid Kurchaninov, Niels Madsen, Scott Menary, Paul Nolan, Konstantin Olchanski, Art Olin, Alex Povilus, Petteri Pusa, Francis Robicheaux, Eli Sarid, Sarah Seif el Nasr, Daniel de Miranda Silveira, Chukman So, James Storey, Robert Thompson, Dirk Peter van der Werf, Jonathan Wurtele, and Yasunori Yamazaki, is available in advance online publication of Nature. ALPHA is supported in part by the National Science Foundation and the U.S. Department of Energy’s Office of Science.