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At work on the ATHENA antihydrogen experiment

The true story of antimatter

The debut of antihydrogen

An atom of antihydrogen consists of an antiproton and a positron (an antielectron), which makes it the simplest antiatom. Unfortunately, this does not make it any easier to produce in the lab. Persuading the antiprotons and positrons to combine together was a challenge that no one had managed to solve until the PS210 at CERN created the first atoms of antihydrogen in 1995.

It was a difficult task both for the physicists and for the operation team at CERN’s Low Energy Antiproton Ring (LEAR) – the main machine used for the experiment. The researchers allowed antiprotons circulating inside LEAR to collide with atoms of a heavy element. Any antiprotons passing close enough to a heavy atomic nucleus could create an electron-positron pair; in a tiny fraction of cases, the antiproton would bind with the positron to make an atom of antihydrogen.

The process was complicated, time-consuming and required a lot of effort but it led to a ground-breaking achievement. When the announcement of the production of 9 antiatoms at CERN was made early in 1996, the news travelled around the world to be reported in newspapers, on radio and on television.

However, the fleeting existence of the antiatoms meant that they could not be used for further studies. Each one existed for only about 40 billionths of a second, travelling at nearly the speed of light over a path of 10 m before it annihilated with ordinary matter.

Antihydrogen en masse

Soon after the PS210 experiment, the study of antimatter at CERN was interrupted. LEAR's operation came to an end in 1996 when the machine had to be converted for use with a future accelerator – the Large Hadron Collider. Another machine called the Antiproton Decelerator was built at CERN to produce and slow down antiparticles. Using this new facility, which came into operation in 2000, two experiments called ATHENA and ATRAP succeeded in producing antihydrogen atoms for the first time in large quantities in 2002.

ATHENA and ATRAP were set up in the late 1990s to produce low-energy antihydrogen atoms for comparison with hydrogen atoms.

In PS210 the antiatoms had been formed in motion close to the speed of light and were too energetic to study. What was needed was a technique to tame the constituents of the antiatoms by trapping, storing and slowing them down. ATHENA and ATRAP achieved this using a novel method that overcame the limitations of the previous techniques.

In September 2002, ATHENA announced the first controlled production of a large number of antihydrogen atoms at low energies and the direct observation of their annihilation. A month later ATRAP announced the first glimpse inside the antiatom.

The ATHENA experiment ended in November 2004 and an experiment called ALPHA has since been set up to continue the research begun by its predecessor.