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Antimatter Atoms Trapped, Stored At CERN Facilities

November 18, 2010

Physicists working at the European Organization for Nuclear Research (CERN) announced on Wednesday that they had successfully trapped and stored antimatter atoms for the first time ever.

Scientists from the Geneva, Switzerland-based research organization, in association with the U.S. Department of Energy’s Lawrence Berkeley National Laboratory and the University of California at Berkeley, trapped a substance known as antihydrogen–the antimatter equivalent of hydrogen.

The international collaboration, known as the Antihydrogen Laser Physics Apparatus or ALPHA experiment, had trapped 38 antihydrogen atoms, each for at least one-tenth of a second, as of early Wednesday evening. Each of the antihydrogen atoms consisted of one single, negatively-charged antiproton orbited by a single, positively-charged anti-electron, also known as a positron.

“For reasons that no one yet understands, nature ruled out antimatter. It is thus very rewarding, and a bit overwhelming, to look at the ALPHA device and know that it contains stable, neutral atoms of antimatter,” Jeffrey Hangst, ALPHA collaboration spokesman and a professor with Aarhus University’s Department of Physics and Astronomy, said in a statement. “This inspires us to work that much harder to see if antimatter holds some secret.”

The researchers published their findings in the journal Nature.

“Antimatter–or the lack of it–remains one of the biggest mysteries of science,” the ALPHA project physicists said in a Wednesday press release. “Matter and its counterpart are identical except for opposite charge, and they annihilate when they meet. At the Big Bang, matter and antimatter should have been produced in equal amounts. However, we know that our world is made up of matter: antimatter seems to have disappeared.”

The ALPHA project was created in 1995 in order to find out what happened to antimatter. Participants opted to focus their efforts on hydrogen because of its simple, well-known structure. In 1995, they produced nine man-made antihydrogen atoms at CERN’s one-of-a-kind low-energy antiproton facility, and in 2002, additional experiments that were conducted showed that it was possible to mass-produce the substance. Wednesday result, they say, is the next step in their quest.

While the antihydrogen atoms produced by CERN are created in a vacuum, they are nonetheless surrounded by normal matter. Normally, the two substances would annihilate each other when they came into contact, but according to the ALPHA project press release, the researchers have been able to use “strong and complex magnetic fields to trap them,” thus preventing them from coming into contact with matter and artificially extending their lifespan.

Containing the antimatter proved to be the biggest challenge for the scientists.

“Trapping antihydrogen proved to be much more difficult than creating antihydrogen,” ALPHA team member Joel Fajans, a scientist at Berkeley Lab’s Accelerator and Fusion Research Division (AFRD) and a professor of physics at UC Berkeley, said. “ALPHA routinely makes thousands of antihydrogen atoms in a single second, but most are too ‘hot’ to be held in the trap. We have to be lucky to catch one.”

In order to create the antihydrogen particles, the accelerators that CERN uses to feed protons into their Large Hadron Collider (LHC) divert a portion of them, forming protons by forcing them into a metal target, a Lawrence Berkeley National Laboratory press release published Wednesday said. The antiprotons that are created are then held in the facility’s Antimatter Decelerator ring, which then delivers some of them to ALPHA and some to a separate antimatter experiment.

“In the ALPHA experiment the antiprotons are passed through a series of physical barriers, magnetic and electric fields, and clouds of cold electrons, to further cool them,” the statement continues. “Finally the low-energy antiprotons are introduced into ALPHA’s trapping region”¦ Meanwhile low-energy positrons, originating from decays in a radioactive sodium source, are brought into the trap from the opposite end.”

Since both particles are charged, they are contained in separate sections of the trap by electric and magnetic fields. When it comes time to bring them together, the antiprotons are “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,” Fajans said. “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.”

By using autoresonance, the ALPHA team members were able to add energy to the antiprotons in such a way that they were able to form low-energy antimatter atoms, which are neutral in charge. In spite of their neutrality, however, the physicists say that their spin and the distribution of the opposite charges of their components allow them to produce momentary magnetism, which allows them to be captured in CERN’s Minimum Magnetic Field Trap’s magnetic and mirror fields.

“Atoms are neutral – they have no net charge – but they have a little magnetic character,” Hangst told BBC News Science and Technology Reporter Jason Palmer on Wednesday. “You can think of them as small compass needles, so they can be deflected using magnetic fields. We build a strong ‘magnetic bottle’ around where we produce the antihydrogen and, if they’re not moving too quickly, they are trapped.”

While thousands of antihydrogen atoms can be produced every second, most have energy levels that are too high, and thus they cannot be caught against the walls of the field trap, the ALPHA physicists claim. Nonetheless, Fajans notes that the group has made serious progress in their experiments.

“We are getting close to the point at which we can do some classes of experiments on the properties of antihydrogen,” he said. “Initially, these will be crude experiments to test CPT symmetry, but since no one has been able to make these types of measurements on antimatter atoms at all, it’s a good start.”

According to Palmer, the next goal of the ALPHA project will be to increase the amount of atoms they are able to produce, as well as extending the amount of time they can last in the trap so that they can be studied more closely.

“What we’d like to do is see if there’s some difference that we don’t understand yet between matter and antimatter,” Hangst told the BBC News reporter. “That difference may be more fundamental; that may have to do with very high-energy things that happened at the beginning of the universe”¦ That’s why holding on to them is so important – we need time to study them.”

Image Caption: An artist’s impression of an antihydrogen atom — a negatively charged antiproton orbited by a positively charge anti-electron, or positron — trapped by magnetic fields. Credit: Katie Bertsche

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