The Underground Area 2 (UA2) experiment was a high-energy physics experiment at the Proton-Antiproton Collider (SppS) — a modification of the Super Proton Synchrotron (SPS) — at CERN. The experiment ran from 1981 until 1990, and its main objective was to discover the W and Z bosons. UA2, together with the UA1 experiment, succeeded in discovering these particles in 1983, leading to the 1984 Nobel Prize in Physics being awarded to Carlo Rubbia and Simon van der Meer. The UA2 experiment also observed the first evidence for jet production in hadron collisions in 1981, and was involved in the searches of the top quark and of supersymmetric particles. Pierre Darriulat was the spokesperson of UA2 from 1981 to 1986, followed by Luigi Di Lella from 1986 to 1990.
Background Around 1968 Sheldon Glashow, Steven Weinberg, and Abdus Salam came up with the electroweak theory, which unified electromagnetism and weak interactions, and for which they shared the 1979 Nobel Prize in Physics. The theory postulated the existence of W and Z bosons, and the pressure on the research community to prove the existence of these particles experimentally was substantial. During the 70s it was established that the masses of the W and Z bosons were in the range of 60 to 80 GeV (W boson) and 75 to 92 GeV (Z boson) — energies too large to be accessible by any accelerator in operation at that time. In 1976, Carlo Rubbia, Peter McIntyre and David Cline proposed to modify a proton accelerator — at that time a proton accelerator was already running at Fermilab and one was under construction at CERN (SPS) — into a proton–antiproton collider, able to reach energies large enough to produce W and Z bosons. The proposal was adopted at CERN in 1978, and the Super Proton Synchrotron (SPS) was modified to occasionally operate as a proton-antiproton collider (SppS).
History On 29 June 1978 the UA1 experiment was approved. Two proposals for a second detector, with the same purpose as UA1, were made the same year. On 14 December 1978, the proposal of Pierre Darriulat, Luigi Di Lella and collaborators, was approved. Like UA1, UA2 was a moveable detector, custom built around the beam pipe of the collider, which searched proton–antiproton collisions for signatures of the W and Z particles. The UA2 experiment began operating in December 1981. The initial UA2 collaboration consisted of about 60 physicists 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 0.9 pb−1. From 1985 to 1987 the SppS 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 13.0 pb−1 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 SppS.
Construction The UA2 experiment was located some 50 meters underground, in the ring of the SPS/SppS 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 the collision point like segments of an orange. Particles ejected from the collision produced showers of secondary particles in the layers of heavy material. These showers passed through layers of plastic scintillators, generating light which was read with photomultiplier by the data collection electronics. The amount of light was proportional to the energy of the original particle. Accurate calibration of the central calorimeter allowed the W and Z masses to be measured with a precision of about 1%.
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