Antimatter atom trapped for first time, say scientists
By Jason Palmer Science and technology reporter, BBC News
Antimatter atoms have been trapped for the first time, scientists say. Researchers at Cern, home of the LHC, have held 38 antihydrogen atoms in place, each for a fraction of a second. While antihydrogen has been produced before, it is instantly destroyed in a flash of light when it encounters normal matter. The team, reporting in Nature, says the ability to study such antimatter atoms will allow previously impossible tests of fundamental tenets of physics. The current "standard model" of physics holds that each particle - protons, electrons, neutrons and a zoo of more exotic particles - has its mirror image antiparticle. The antiparticle of the electron, for example, is the positron, and is used in an imaging technique of growing popularity known as positron emission tomography. However, one of the great mysteries in physics is why our world is made up overwhelmingly of matter, rather than antimatter; the laws of physics make no distinction between the two and equal amounts should have been created at the Universe's birth. Producing antimatter particles like positrons and antiprotons has become commonplace in the laboratory, but assembling the particles into antimatter atoms is far more tricky. That was first accomplished by two groups in 2002. But handling the "antihydrogen" - bound atoms made up of an antiproton and a positron - is trickier still because it must not come into contact with anything else. While trapping of charged normal atoms can be done with electric or magnetic fields, trapping antihydrogen atoms in this "hands-off" way requires a very particular type of field. "Atoms are neutral - they have no net charge - but they have a little magnetic character," explained Jeff Hangst of Aarhus University in Denmark, one of the collaborators on the Alpha antihydrogen trapping project.
"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," he told BBC News. Such sculpted magnetic fields that make up the magnetic bottle are not particularly strong, so the trick was to make antihydrogen atoms that didn't have much energy - that is, they were slow-moving. The team proved that among their 10 million antiprotons and 700 million positrons, 38 stable atoms of antihydrogen were formed, lasting about two tenths of a second each. Next, the task is to produce more of the atoms, lasting longer in the trap, in order to study them 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," Professor Hangst said. "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." Gerald Gabrielse of Harvard University led one of the groups that in 2002 first produced antihydrogen, and first proposed that the "magnetic bottle" approach was the way to trap the atoms. "I'm delighted that it worked as we said it should," Professor Gabrielse told BBC News. "We have a long way to go yet; these are atoms that don't live long enough to do anything with them. So we need a lot more atoms and a lot longer times before it's really useful - but one has to crawl before you sprint. Professor Gabrielse's group is taking a different tack to prepare more of the antihydrogen atoms, but said that progress in the field is "exciting". "It shows that the dream from many years ago is not completely crazy."
