In September 1995, Prof. Walter Oelert and an international team from Jülich IKP-KFA, Erlangen-Nuernberg University, GSI Darmstadt and Genoa University succeeded for the first time in synthesising atoms of antimatter from their constituent antiparticles. Nine of these atoms were produced in collisions between antiprotons and xenon atoms over a period of three weeks. Each one remained in existence for about forty billionths of a second, travelled at nearly the speed of light over a path of ten metres and then annihilated with ordinary matter. The annihilation produced the signal which showed that the anti-atoms had been created.
Ordinary atoms consist of a number of electrons in orbit around an atomic nucleus. The hydrogen atom is the simplest atom of all; its nucleus consists of a proton, around which a single electron circulates. The recipe for anti-hydrogen is very simple - take one antiproton, bring up one anti-electron, and put the latter into orbit around the former - but it is very difficult to carry out as antiparticles do not naturally exist on earth. They can only be created in the laboratory. The experimenters whirled previously created antiprotons around the CERN* Low Energy Antiproton Ring (LEAR), passing them through a xenon gas jet each time they went around - about 3 million times each second. (see scheme of the experiment) Very occasionally, an antiproton converted a small part of its own energy into an electron and an anti-electron, usually called a positron, while passing through a xenon atom. In even rarer cases, the positron's velocity was sufficiently close to the velocity of the antiproton for the two particles to join - creating an atom of anti-hydrogen (see diagram of the principle) .
Three quarters of our universe is hydrogen and much of what we have learned about it has been found by studying ordinary hydrogen. If the behaviour of anti-hydrogen differed even in the tiniest detail from that of ordinary hydrogen, physicists would have to rethink or abandon many of the established ideas on the symmetry between matter and antimatter. Newton's historic work on gravity was supposedly prompted by watching an apple fall to earth, but would an "anti-apple" fall in the same way? It is believed that antimatter "works" under gravity in the same way as matter, but if nature has chosen otherwise, we must find out how and why.
The next step is to check whether anti hydrogen does indeed "work" just as well as ordinary hydrogen. Comparisons can be made with tremendous accuracy, as high as one part in a million trillion, and even an asymmetry on this tiny scale would have enormous consequences for our understanding of the universe. To check for such an asymmetry would mean holding the anti-atoms still, for seconds, minutes, days or weeks. The techniques needed to store antimatter are under intense development at CERN. New experiments are currently being planned, to capture antimatter in electrical and magnetic bottles or traps allowing for high precision analysis.
The first ever creation of atoms of antimatter at CERN has opened the door to the systematic exploration of the antiworld.
1. CERN, the European Laboratory for Particle Physics, has its headquarters in Geneva. At present, its Member States are Austria, Belgium, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Italy, Netherlands, Norway, Poland, Portugal, the Slovak Republic, Spain, Sweden, Switzerland and the United Kingdom. Israel, Japan, the Russian Federation, Turkey, the European Commission and Unesco have observer status.