The Antiproton Decelerator (AD) is a unique machine that produces low-energy antiprotons for studies of antimatter, and “creates” antiatoms. The Decelerator produces antiproton beams and sends them to the different experiments.

A proton beam that comes from the PS (Proton Synchrotron) is fired into a block of metal. These collisions create a multitude of secondary particles, including lots of antiprotons. These antiprotons have too much energy to be useful for making antiatoms. They also have different energies and move randomly in all directions. The job of the AD is to tame these unruly particles and turn them into a useful, low-energy beam that can be used to produce antimatter.

The antiprotons, which emerge from the block at diverging angles, are focused before they reach the AD. Only a fraction of them have the right energy to be injected into and stored in the AD.

The AD is a ring composed of bending and focussing magnets that keep the antiprotons on the same track, while strong electric fields slow them down. The spread in energy of the antiprotons and their deviation from their track is reduced by a technique known as “cooling”. Antiprotons are subjected to several cycles of cooling and deceleration until they are slowed down to around a tenth of the speed of light. They are then ready to be ejected into the antimatter experiments.

ELENA (Extra Low ENergy Antiproton) is a new deceleration ring that will soon be commissioned. Coupled with the AD, this synchrotron, with a circumference of 30 metres, will slow the antiprotons even more, reducing their energy by a factor of 50, from 5.3 MeV to just 0.1 MeV. An electron cooling system will increase the beam density. The number of antiprotons that can be trapped will be increased by a factor of 10 to 100, improving the efficiency of the experiments and paving the way for new experiments.

Installed in 2000, the AD made the headlines in 2002 when large numbers of antihydrogen atoms were produced for the first time. Initial attempts were made to store antiatoms for a long enough time to be able to measure their characteristics. In 2011, an experiment announced that it had produced and trapped antihydrogen atoms for sixteen minutes, which was long enough to be able to study their properties in detail. The following year, the first measurement of the antihydrogen spectrum was published. Since 2010, the AD experiments have published numerous measurements of antimatter characteristics, comparing them to those of matter.

Currently the AD serves several experiments that are studying antimatter and its properties: ALPHAASACUSAATRAP and BASE. Two other experiments, AEGIS and GBAR, are preparing to study the effects of gravity on antimatter. GBAR will be the first experiment to use antiprotons prepared by ELENA, the new decelerator.