Thousands of cold anti-atoms produced at CERN

An international team of physicists working at the Antiproton Decelerator (AD) facility at CERN has announced the first controlled production of large numbers of antihydrogen atoms at low energies. After mixing cold clouds of trapped positrons and antiprotons – the antiparticles of the familiar electron and proton – under closely monitored conditions, the ATHENA collaboration has identified antihydrogen atoms, formed when positrons bind together with antiprotons. The results are published online today by the journal Nature.

Says Professor Luciano Maiani, Director General of CERN, “The controlled production of antihydrogen observed in ATHENA is a great technological and scientific event. Even more so because ATHENA has produced antihydrogen in unexpectedly abundant quantities. I’d like also to recognise the contribution of the ATRAP experiment at CERN, which has pioneered the technology of trapping cold antiprotons and positrons, an essential step towards the present discovery.”

The ATHENA experiment, which is run by a collaboration of 39 scientists from 9 different institutions worldwide, saw its first clear signals for antihydrogen in August – appropriately, the 100th anniversary of the birth of theorist Paul Dirac who predicted the existence of antimatter in the late 1920s. Says ATHENA spokesman, Rolf Landua,”The experiment is a major milestone in antimatter science and an important first step on the road to high precision comparisons of hydrogen and antihydrogen. Such measurements will provide information vital to our understanding of the Universe and in particular why nature has a preference for matter over antimatter.”

The method ATHENA uses overcomes the two main limitations of previous experiments both at CERN and at Fermilab in the US, which produced only a few anti-atoms per day with velocities close to the speed of light. First, the AD takes high energy antiprotons and slows them down to the leisurely pace – by CERN’s standards – of a tenth of the speed of light. ATHENA then traps the antiprotons in a “cage” created by electromagnetic fields, and reduces their velocity further to a few millionths of the speed of light. The ATHENA apparatus captures and slows down – or “cools” – about 10,000 antiprotons from each bunch that arrives from the AD. The next stage is to mix them with about 75 million cold positrons. These are collected from the decay of a radioactive isotope, then caught within a second trap, and finally transferred to a third, “mixing” trap. It is here that cold – that is, very slow – antihydrogen atoms may form.

Central to ATHENA’s observations is the antihydrogen annihilation detector, which surrounds the trap where the antiprotons and positrons are mixed. When a positron and an antiproton bind together to form a neutral antihydrogen atom, it escapes the trapping electromagnetic fields, which are set up by metal electrodes. The anti-atom then strikes one of the electrodes, and the positron and antiproton annihilate separately, with an electron and a proton, respectively, in the surface of the metal.

The detector provides unambiguous evidence for antihydrogen by detecting the simultaneous annihilations of the antiproton and the positron, which occur at the same time and at the same position. ATHENA finds that several anti-atoms per second are produced on average during the procedure that mixes the positrons and antiprotons. So far the experiment has produced about 50,000 antihydrogen atoms.

ATHENA is one of two experiments set up to search for cold antihydrogen at the AD. Last year the ATRAP experiment was the first to use cold positrons to cool antiprotons. The experiment also successfully confined both ingredients of cold antihydrogen in the same trap structure. This simultaneous trapping of positrons and antiprotons was first demonstrated by TRAP, the predecessor to ATRAP, which operated on the Low Energy Antiproton Ring (LEAR) at CERN.

These breakthroughs at CERN are important milestones on the way to trapping, accumulating and cooling antihydrogen. Cold antihydrogen will be a new tool for precision studies in a broad range of science. Most fundamental will be the comparison of the interaction of hydrogen and antihydrogen with electromagnetic and gravitational fields. Any difference between matter and antimatter, however small, would have profound consequences for our fundamental understanding of Nature and the Universe.