Unlike most experiments at CERN, the £3.2 million Antimatter Decelerator (AD) is designed to slow down particles rather than accelerate them. The antiprotons are created when high-energy protons from the lab's Proton Synchrotron strike an iridium target. The antiprotons are siphoned off and directed towards the AD, a 188m circumference ring, where they are focussed into a beam and slowed down.

The deceleration process causes the spread of energies in the beam to increase, and so the beam must be stabilized with a variety of stochastic and electron cooling techniques. The beam momentum is reduced from 3.57 GeV/c to about 100 MeV/c, which is low enough for detailed antimatter experiments, in several stages. The deceleration process takes about one minute and has an efficiency of about 25%.

Two of the experiments at the decelerator, ATRAP and ATHENA, will study antihydrogen, while the ASACUSA experiment will look at the 'atomcules' produced when an electron in a helium atom is replaced with an antiproton.

Antihydrogen was first produced with high-energy antiprotons at CERN in 1995, but the antiatoms did not survive long enough for them to be studied in experiments. A source of low energy antihydrogen would open up a number of exciting research possibilities. "For the first time we will be able to isolate and trap antihydrogen in a way which will enable precise analysis," said CERN spokesman Neil Calder.

The ATRAP experiment is designed to produce 'cold' antihydrogen atoms by combining the antiprotons with positrons in a single Penning trap. The ATHENA project will try a slightly different approach, firing the antiprotons through a cloud of positrons in the hope that some will stick together. A major aim of both experiments is to carry out a detailed comparison of the atomic structures of hydrogen and antihydrogen. Any differences could help explain why our universe is dominated by matter.