The UA2 detector on the Super Proton Synchrotron scrutinized proton-antiproton collisions for traces of W and Z particles

Underground Area 2 (UA2) was a particle detector on the Super Proton Synchrotron (SPS). It ran from 1981 until 1990, when the SPS was used as a proton-antiproton collider, searching the collisions for traces of the W and Z particles.

Two moveable detectors - UA1 and UA2 - were custom built around the SPS beam pipe for use during proton-antiproton running. They could be rolled back after periods of data taking, so that the SPS could revert to fixed-target operation.

The UA2 experiment was approved in December 1978. A previous experiment called UA5 operated for a short time earlier in 1981 and was moved to make way for UA2, which saw its first collisions in December 1981. Unlike UA1, UA2 was not a multipurpose detector; it had a more limited scope. The focus was on the calorimeters – detectors that measured the energy particles lost as they passed through. UA2 had electromagnetic and hadronic calorimeters to detect electrons and hadrons, but could not measure particle charges except for limited regions where the W decay asymmetry was maximal. There was no muon detector.

UA2  provided the most accurate measurements of the W and Z masses and it was state-of-the-art at detecting particle jets - sprays of hadrons and other particles that form when a quark, gluon or antiquark is ejected from the collisions.

At a seminar on 22 January 1983, UA2 physicist Luigi Di Lella announced to a packed CERN auditorium that the UA2 detector had recorded four events that were candidates for a W boson. This brought the combined number of candidate events seen by UA1 and UA2 up to 10. Three days later, on 25 January, CERN shared news of the discovery of the W boson with the world.

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