The Low Energy Antiproton Ring (LEAR) decelerated and stored antiprotons for experiments. It was built in 1982 and operated until 1996, when it was converted into the Low Energy Ion Ring (LEIR), which provides lead-ion injection for the Large Hadron Collider (LHC).

Under the LEAR programme, four machines – the Proton Synchrotron (PS), the Antiproton Collector (AC), the Antiproton Accumulator (AA), and LEAR – worked together to collect, cool and decelerate antiprotons for use in experiments. Protons from the PS created antiprotons in collisions with a fixed target. Then the AC and the AA worked to catch antiprotons in sufficient numbers, and LEAR, the final element in the chain, passed the antiprotons on to experiments.

When the antiproton machines were closed down in 1996 to free resources for the LHC, a number of researchers wished to continue their experiments with slow antiprotons. The CERN council asked the PS division to investigate a low-cost way to provide the necessary low-energy beams. The resulting design report for a new machine – the Antiproton Decelerator (AD) – was approved on 7 February 1997. The AD started operation in 2000 and is still the main provider of antiparticles at CERN today.