Entering the world's premier antimatter factory

I DESCEND into the bowels of building 193 to particle physics' version of Bizarro World. Before me lies what looks like a miniature Large Hadron Collider, with superconducting magnets surrounding a 190-metre-long particle track. Instead of accelerating particles, this contraption decelerates them - the only one at CERN built to make particles sluggish.

I am at the premier factory for making the most expensive atoms, by weight, on the planet. Here, there is no need for speed because the four experiments in this building are not trying to create collisions powerful enough to give rise to hypothetical entities such as the Higgs boson. Instead, they are trying to unlock the secrets of antimatter, which means slowing antiparticles down enough to carry out experiments - often on a single atom. "With the LHC you have sledge-hammer precision; we need scalpel-precision to carry out experiments on single atoms," says physicist Jeffrey Hangst.

Antiparticles are notoriously slippery test subjects. With the same mass but the opposite charge to ordinary particles, they disappear in a puff of energy whenever they meet normal matter, which makes up everything we see. That means antiparticles have to be painstakingly made and captured for each test.

The pay-off stands to be huge. Ordinary matter and antimatter are expected to behave the same way - responding identically to gravity, for instance. If they don't, it could be the calling card of exotic new forces, beyond those described by the standard model and Einstein's general relativity. "How do you get famous?" asks Michael Doser, spokesman for an upcoming experiment called AEGIS to test how antimatter reacts to gravity. "Go after Einstein - the biggest game in town."

Indeed, competition for such glory is fierce, with each of the four teams marking their territory with flags from team members' universities. "There is definitely more competition than collaboration," admits Hangst.

All four aim to carry out surgically precise measurements on antihydrogen, made up of an antiproton and a positron, the anti-electron. Protons produced elsewhere at CERN are smashed into a metal target to create the antiprotons, which are then electromagnetically channelled into the Antiproton Decelerator - the "mini-LHC" in the basement of building 193. From there, they are sent up through pipes to the four experiments above, which combine them with positrons to create antihydrogen atoms.

Other than the gravity-testing AEGIS, the other three experiments - ALPHA, ATRAP and ASACUSA - all aim to see if antihydrogen absorbs and emits light at the same wavelengths as hydrogen. ALPHA recently made the first such spectral measurement, but it wasn't precise enough to detect any differences.

Still, any progress is celebrated. Because neutral antihydrogen only responds weakly to electromagnetic fields, the atoms have to be moving very slowly to be captured for study. "We make about 6000 antihydrogen atoms at a time but only catch one or two," says Hangst, who is ALPHA team leader. "Everyone still rushes over with bated breath to see if we have anything."

That's bad news for anyone dreaming of cadging a ride on an antimatter-fuelled spacecraft, but it's a start.

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