Upping the Anti: Antimatter Atoms Stored in Trap
BERKELEY, Calif., June 7, 2011 — A total of 309 antihydrogen atoms have been created and stored for as long as 1000 seconds (almost 17 min), with an indication of much longer storage times as well. The team involved in the collaboration said that adding lasers, which are essential for cooling antihydrogen atoms, is a goal it hopes to accomplish by 2012.
The international team, known as the Alpha Collaboration, includes scientists from the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California at Berkeley, including physics professors Joel Fajans and Jonathan Wurtele of Berkeley Lab's Accelerator and Fusion Research Division (AFRD).
In an antihydrogen atom (top), a positively charged antielectron, or positron, orbits a negatively charged antiproton. This configuration is the mirror image of an ordinary hydrogen atom (bottom). (Images: Chukman So, Wurtele Research Group)
"We've trapped antihydrogen atoms for as long as 1000 seconds, which is forever [in the world of high-energy particle physics]," Fajans said.
The Alpha team is working to build a new antihydrogen trap with "the hope that by 2012 we will have a new trap with laser access to allow spectroscopic experiments on the antiatoms," he said.
Their study reports that in a series of measurements last year, the team trapped 112 antiatoms for times ranging from one-fifth of a second to 1000 seconds, or 16 minutes and 40 seconds.
Since the experiment first successfully trapped antihydrogen atoms in 2009, the researchers have captured 309.
"We'd prefer being able to trap a thousand atoms for a thousand seconds, but we can still initiate laser and microwave experiments to explore the properties of antiatoms," Fajans said.
This is an artistic representation of the Alpha neutral antimatter trap, suggesting the nature of the Alpha apparatus as a container for antihydrogen.
In November 2010, Fajans, Wurtele and the Alpha team reported their first data on trapped antihydrogen: 38 antiatoms trapped for more than one-tenth of a second each. They succeeded in capturing an antiatom in only about one in 10 attempts, however.
Toward the end of last year's experiments, they captured an antiatom in nearly every attempt and kept the antiatoms in the trap as long as they wanted. Realistically, trapping for 10 to 30 minutes will be sufficient for most experiments, as long as the antiatoms are in their lowest energy state, or ground state.
"These antiatoms should be identical to normal-matter hydrogen atoms, so we are pretty sure all of them are in the ground state after a second," Wurtele said.
"These were likely the first ground-state antiatoms ever made," Fajans added.
Antimatter is a puzzle because it should have been produced in amounts equal to normal matter during the Big Bang that created the universe 13.6 billion years ago. Today, however, there is no evidence of antimatter galaxies or clouds, and antimatter is seen rarely and for only short periods, for example during some types of radioactive decay before it annihilates in a collision with normal matter.
Hence the desire to measure the properties of antiatoms to determine whether their electromagnetic and gravitational interactions are identical to those of normal matter. One goal is to check whether antiatoms abide by CPT symmetry, as do normal atoms. CPT (charge-parity-time) symmetry means that a particle would behave the same way in a mirror universe if it had the opposite charge and moved backward in time.
"Any hint of CPT symmetry breaking would require a serious rethink of our understanding of nature," said Jeffrey Hangst of Aarhus University in Denmark, spokesman for the Alpha experiment. "But half of the universe has gone missing, so some kind of rethink is apparently on the agenda."
Artist's image of the Alpha trap that captured and stored antihydrogen atoms.
A principal component of Alpha's atom trap is a superconducting octupole magnet proposed and prototyped in Berkeley Lab's AFRD. It takes Alpha about 15 minutes to make and capture atoms of antihydrogen in its magnetic trap.
Alpha captures antihydrogen by mixing antiprotons from CERN's Antiproton Decelerator with positrons – antielectrons – in a vacuum chamber, where they combine into antihydrogen atoms. The cold neutral antihydrogen is confined within a magnetic bottle, taking advantage of the tiny magnetic moments of the antiatoms. Trapped antiatoms are detected by turning off the magnetic field and allowing the particles to annihiliate with normal matter, which creates a flash of light.
"So far, the only way we know whether we've caught an antiatom is to turn off the magnet," Fajans said. "When the antiatom hits the wall of the trap, it annihilates, which tells us that we got one. In the beginning, we were turning off our trap as soon as possible after each attempt to make antiatoms, so as not to miss any."
Because the confinement depends on the antihydrogen's magnetic moment, if the spin of the antiatom flips, it is ejected from the magnetic bottle and annihilates with an atom of normal matter. This gives the experimenters an easy way to detect the interaction of light or microwaves with antihydrogen, because photons at the right frequency make the antiatom's spin flip up or down.
Though the team has trapped up to three antihydrogen atoms at once, the goal is to trap even more for long periods of time to achieve greater statistical precision in the measurements.
"It may not sound exciting, but it's the first experiment done on trapped antihydrogen atoms," Wurtele said. "This summer, we're planning more experiments, with microwaves. Hopefully, we will measure microwave-induced changes of the atomic state of the antiatoms."
For more information, visit: www.lbl.gov
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