Antihydrogen Antics: Research

With the discovery of the positron and antiproton, physicists dreamed of combining them to make anti-hydrogen, the simplest atom made of antimatter. Examining how anti-hydrogen absorbs and emits light would permit a stringent test of particle theory, which says that anti-hydrogen’s energy spectrum should be identical to that of ordinary hydrogen.

The particle decelerator in the ATHENA project at CERN (photo courtesy of CERN)

An atom of anti-hydrogen annihilates to form two gamma rays and two pions. From observations of these annihilation products, physicists constructed this computer image. (image courtesy of CERN)

Physicists first produced antihydrogen at the CERN and Fermilab particle accelerators in the mid-1990s. A beam of antiprotons smashed into a target, creating many new particles, including electron-positron pairs. A few antiprotons and positrons did combine each day, but the best the experimenters could do was detect the antimatter atoms as they raced away at almost the speed of light.

But physicists want to study the antihydrogen, not just detect it, and to do this they need a large, confined sample. The challenges start with the production of the antiprotons, which requires extremely high energies, so once produced, these particles must be decelerated in what is essentially a particle accelerator run backwards. (See photo) Then the antiprotons enter a series of electric and magnetic traps, cooling them considerably more. In a parallel part of the experiment, positrons from radioactive decay are themselves trapped and cooled.

When positrons and antiprotons finally meet, in yet another trap, some combine to form antihydrogen. Since atoms, including those of antimatter, are electrically neutral, they no longer respond to the electric and magnetic fields and simply fall out of the trap and into annihilation on one of the metal electrodes. From the annihilation products, physicists detect atoms of antihydrogen, as shown in the computer image. The experiment yields about 50,000 antihydrogen atoms, starting from about 1.5 million antiprotons. This sample size is still too small for spectroscopy, but future experiments will increase the yield substantially.

Beyond the study of antimatter for its own sake, these investigations could help cosmologists understand the early history of the universe. The Big Bang should have produced matter and antimatter in equal amounts, but the observed universe seems to be made of only matter. The answer to the question of the missing antimatter could lie in some yet-to-be-discovered difference between hydrogen and antihydrogen.