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At CERN, atoms of antihydrogen that stick around: physicists trap antiparticles almost long enough to study.

Researchers are a step closer to the Angels & Demons scenario dreamed up by novelist Dan Brown, in which a rogue priest tries to destroy the Vatican with a vial of antimatter.

In reality, the amount of antimatter created to date wouldn't release enough energy to heat a pot of coffee. But physicists at CERN, the European particle physics laboratory near Geneva, have now managed to make and hold dozens of antihydrogen atoms for a fraction of a second, far longer than ever before.

The work, reported online in Nature November 17, is a significant step toward making antimatter stick around long enough to be able to study how it differs from ordinary matter.

"In 10 years people will forget Dan Brown, but we'll be in the textbooks," says team spokesman Jeffrey Hangst, a physicist at Aarhus University in Denmark.

Antimatter, whose existence was predicted by physicist Paul Dirac in 1931, is made of particles with electric charges opposite those of ordinary matter. While a hydrogen atom is made of a positively charged proton and a negatively charged electron, an antihydrogen atom is made of a negatively charged antiproton and a positively charged positron. When matter and antimatter meet, they annihilate.


Theory suggests that equal amounts of matter and antimatter should have been formed in the Big Bang, nearly 14 billion years ago, and physicists have long puzzled over why matter prevails. Several experiments at CERN seek to explain matter's predominance using an "antiproton decelerator"--a much smaller cousin to the more famous Large Hadron Collider--that slows the particles down to about a tenth the speed of light.

Physicists at the lab first made a few fleeting atoms of antihydrogen in 1995. In 2002 two teams reported ways to make lots of antihydrogen atoms at low energies--crucial for making measurements.

The new results come from Hangst's ALPHA experiment, which cools a stream of antiprotons into a cloud of about 40,000 particles. The researchers then nudge that cloud very gently into a couple of million positrons chilled to 40 kelvins (40 degrees Celsius above absolute zero). About one time out of 10, an antiproton and a positron combine to make an antihydrogen atom.

For roughly every 100,000 antihydrogen atoms made, researchers managed to trap just one of them using strong magnetic fields. The Nature paper reports 38 such trapped antiatoms, which were held for more than a sixth of a second before they escaped and annihilated themselves against the matter in the sides of the container.

If scientists can keep more antihydrogen around for longer, they can do studies such as spectral measurements to reveal how the material's internal energetics differs from those of ordinary hydrogen. "It's so exciting because now we can subject antihydrogen to anything anyone has ever done with hydrogen in the past," says Hangst. "There are 100-odd years of atomic physics that we can learn how to do again with antiatoms."

Antihydrogen looks to be a growing field. Two other new antihydrogen experiments are just getting underway at CERN. One of them, called ASACUSA, recently made its first antihydrogen atoms using a device called a cusp trap. This approach could allow scientists to make the first supersensitive measurements of antihydrogen at microwave wavelengths, the team reports in a paper accepted for publication in Physical Review Letters. ASACUSA plans to let the atoms leak out of the traps and study them in flight.

The most recent experiment, called AEGIS, hasn't made antihydrogen yet but plans to eventually study gravity's effects on antimatter.

ALPHA's long-standing competitor is another experiment called ATRAP, which works about five meters down the antiproton beam. It is currently focusing on making much bigger, colder blobs of antiprotons that can then be combined with positrons to make antihydrogen.

In a paper published online in Physical Review Letters November 16, the ATRAP group reports centrifugally separating antiprotons from electrons, which could provide a new way to isolate low-energy antiprotons for making antihydrogen. "We have up to 3 million antiprotons whose temperature is less than 6 kelvin," says team leader Gerald Gabrielse of Harvard University.

In 1987, Gabrielse proposed that it might be possible to confine and study cold antihydrogen atoms. "I'm naturally delighted at the news that a few atoms have been fleetingly confined," he says.

Hangst's group is now pushing to get temperatures (and thus particle energies) lower. CERN's antiproton beam has been shut down temporarily but will start up again in May for its next run. And if funding comes through, a planned upgrade in the next couple of years would increase the antiproton supply a hundredfold. Right now, scientists can get as many as 5 million antiprotons per hour.

Antihydrogen: A how-to guide Trapping antihydrogen long enough to study it requires merging clouds of antiprotons and positrons at extremely low temperatures. Some of the particles will pair up to form antihydrogen atoms, which can be trapped for a fraction of a second by a powerful magnetic field before they make their way to the walls of the container and annihilate on contact.


The outer shell of the ALPHA experiment detects particles generated when antihydrogen atoms annihilate, employing the same material that accelerators like the Large Hadron Collider use to detect some of the debris from particle collisions.


When a pair of positrons collide inside the mixed cloud, there is a chance that one of them will then bind to a nearby antiproton to form an atom of antihydrogen.


Inside the annihilation detector is a cylinder-shaped superconducting electromagnet designed to trap antihydrogen atoms in a "magnetic minimum"--the magnetic equivalent of trapping a marble in the bottom of a concave dish.


With the superconducting magnet turned on, antihydrogen atoms are trapped in the cylinder much like a marble settling at the bottom of a concave dish (brown).


Still deeper is a cylindrical set of electrodes (gold) that holds clouds of charged antiprotons and positrons in place at a temperature of 40 kelvins. The electrodes produce an electric field that brings antiprotons and positrons together.


With the magnet turned off, the atoms drift into the walls of the apparatus, where their destruction is sensed by the annihilation detector--proof that they did in fact exist.
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Title Annotation:STORY ONE; European Organization for Nuclear Research
Author:Witze, Alexandra
Publication:Science News
Geographic Code:4EXSI
Date:Dec 18, 2010
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