CERN experiment provides first glimpse inside cold antihydrogen

The ATRAP1 experiment at the Antiproton Decelerator at CERN2 has detected and measured large numbers of cold antihydrogen atoms. Relying on ionization of the cold antiatoms when they pass through a strong electric field gradient, the ATRAP measurement provides the first glimpse inside an antiatom, and the first information about the physics of antihydrogen. The results have been accepted for publication in Physical Review Letters.

ATRAP's technique relies on trapping positrons between two bunches of antiprotons in a nested trap structure. The positrons are used to cool the antiprotons, and when the positrons and antiprotons reach a similar temperature, some combine to form antihydrogen atoms (a positron orbiting an antiproton nucleus). ATRAP team member Walter Oelert from the Forschungszentrum JŸlich in Germany, who led the experiment that first observed antihydrogen at CERN, explains "In 1996 we produced only a few atoms of antihydrogen at a velocity close to the speed of light, which is equivalent to a temperature 100,000 times that of the inner part of the Sun. You can imagine that this material is too hot to handle. Now we have antihydrogen in much larger quantities as cold as only a few degrees above absolute zero."

Being electrically neutral, the antiatoms drift out of the trap. Those that move along the axis of the apparatus soon find themselves traversing a strong electric field that strips off the positrons, thereby allowing the negatively charged antiprotons to be trapped and counted. "This measurement is completely background free," explains ATRAP spokesperson Jerry Gabrielse of Harvard University, "since the only way that a signal is detected is if an antiproton escapes the nested trap in the form of neutral antihydrogen atoms."

The ATRAP team has measured the field needed to ionize the antihydrogen atoms. The result shows that the antiatoms are formed in highly excited states. This is being interpreted as pointing to a three-body recombination scheme where a third body carries away the energy and momentum liberated by the antiatom's formation. The ATRAP method has allowed the first measurement of the physics of antihydrogen, and is a significant step on the way to the precision measurements that will allow matter-antimatter comparisons to be made. Jerry Gabrielse says : "Our long term goal is to try and look very precisely at this antiatom, and by comparing the world's simplest antimatter atom and the world's simplest matter atom to make a very fundamental test of basic physics theories."

This news comes shortly after ATHENA, another experiment at the Antiproton Decelerator, announced its observation of cold antihydrogen. Using a completely different detection technique to ATHENA, and by providing the first glimpse into the internal structure of antihydrogen, ATRAP has shown that CERN researchers are firmly on the road to understanding the first entry in the periodic table of the anti-elements. Both ATHENA and ATRAP use similar techniques for trapping the ingredients of antihydrogen, developed over many years by Gabrielse's team. The fact that they use different detection methods reinforces the result, and is a good omen for future studies of antihydrogen at CERN.

In a second paper submitted to Physical Review Letters (and now being considered for publication), ATRAP reports an even more efficient method for producing antihydrogen, in which antiprotons are driven into repeated collisions with cold positrons. The production rate is high enough that for the first time a distribution of antihydrogen states is measured.

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1. The ATRAP collaboration's participating institutions: Department of Physics, Harvard University, Cambridge, MA 01238, USA IKP, Forschungzentrum, Jülich GmbH, 52425 Jülich, Germany CERN, CH-1211 Geneva 23, Switzerland Max-Planck-Institut für Quantenoptik, Hans-Kopfermnann-Strasse 1, 85748 Garching, Germany York University, Department of Physics and Astronomy, Toronto, Ontario M3J 1P3, Canada
2. CERN, the European Organization for Nuclear Research, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, Bulgaria, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, Slovakia, Spain, Sweden, Switzerland and the United Kingdom. Israel, Japan, the Russian Federation, the United States of America, Turkey, the European Commission and Unesco have observer status.

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