from Bern, CERN, Copenhagen, Orsay, Pavia and Saclay. From 1981 to 1985, the UA1 and UA2 experiments collected data corresponding to an integrated luminosity of approximately . From 1985 to 1987 the SpS was upgraded, and the luminosity of the machine increased by a factor 10 compared to the previous performance. The UA2 sub-detectors were also upgraded, making the detector hermetic, which increased its ability to measure missing transverse energy. The second experimental phase ran from 1987 to 1990. Groups from Cambridge, Heidelberg, Milano, Perugia and Pisa joined the collaboration, which grew to about 100 physicists. During this phase, UA2 accumulated data corresponding to an integrated luminosity of in three major running periods. After nearly ten years of operation, the UA2 experimental program stopped running at the end of 1990. Components and operation The UA1 and UA2 experiments recorded data during proton–antiproton collision operation and moved back after periods of data taking, so that the SPS could revert to fixed-target operation. UA2 was moved on air cushions when removed from the beam pipe of the SpS. Construction The UA2 experiment was located some 50 meters underground, in the ring of the SPS/SpS accelerator, and was housed in a big cavern. The cavern was large enough to house the detector, provide room for it to be assembled in a "garage position" without shutting down the accelerator and to where it was also moved back after periods of data taking. The accelerator could therefore revert to fixed-target operation, after periods of operating as a collider. Detectors The UA1 and the UA2 experiments had many things in common; they were both operating on the same accelerator and both had the same objective (to discover the W and Z bosons). The main difference was the detector design; UA1 was a multipurpose detector, while UA2 had a more limited scope. UA2 was optimized for the detection of electrons from W and Z decays. The emphasis was on a highly granular calorimeter – a detector measuring how much energy particles deposit – with spherical projective geometry, which also was well adapted to the detection of hadronic jets. Charged particle tracking was performed in the central detector utilising a combination of multi-wire proportional chambers and drift chambers and hodoscopes. Energy measurements were performed in the calorimeters. Unlike UA1, UA2 had no muon detector. The calorimeter had 24 slices, each weighing 4 tons. These slices were arranged around