The anti-hydrogen atoms were produced from antiprotons from CERN’s Antiproton Decelerator and positrons from a radioactive sodium-22 source. The positrons, which were trapped between sets of antiprotons in a “Penning” trap, cooled the antiprotons. When both reach a similar temperature some combine to form anti-hydrogen atoms, consisting of a positron orbiting an antiproton nucleus. These anti-atoms, which are electrically neutral, drift out of the trap. Any anti-hydrogen atoms moving along the axis of the apparatus traverse a strong electric field that removes the positron from the anti-atom. This “field-ionisation” technique allows the resulting negatively charged antiprotons to be trapped and counted.

Using this technique the researchers were able to produce nearly 170 000 cold anti-hydrogen atoms. This means that a remarkable 11% of the antiprotons in the Penning trap formed anti-hydrogen atoms. This compares well with previous experiments performed at CERN, by researchers on the ATHENA collaboration, using a similar trapping technique they produced about 50 000 anti-hydrogen atoms two months ago.

ATRAP’s field-ionisation technique also gives information about the internal states of the anti-hydrogen atoms, showing that the principal quantum number n is between about 43 and 55 (where n=1 corresponds to the ground state). By changing the strength of the ionising electric field the researchers hope to discover more about the internal state of the anti-hydrogen atoms, and to learn how to de-excite them to the ground state. This knowledge will be essential because hydrogen atoms and anti-atoms can only be trapped if they are in their ground state.

This high rate of production, and the fact that the anti-atoms are formed in highly excited states, suggests that the anti-hydrogen atoms are formed in three-body collisions between two positrons and an antiproton.

The ATRAP collaboration, which includes researchers from the US, Switzerland, Germany and Canada first demonstrated the cooling of antiprotons with positrons in a Penning trap last year. Since then they have carried out more detailed studies of this cooling process to ensure that the antiproton loss observed during positron cooling is indeed due to the formation of anti-hydrogen and not other mechanisms. The team is confident that every recorded event comes from the production of an anti-hydrogen atom and that their measurements are free of background.

The ultimate goal of the experiments will be to trap cold anti-hydrogen atoms and study their spectra in detail. Comparing the spectra of anti-hydrogen with hydrogen, and studying the transition from the n=2 to the n=1 state in particular, will give researchers new insights into the differences between matter and antimatter.