ЦЕРН-дегі W және Z бозондарының ашылуы: UA2 эксперименті

Particle physics experiment at CERNThe Underground Area 2 (UA2) experiment was a high energy physics experiment at the Proton Antiproton Collider (SpS) — 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.

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 (SpS).

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. After nearly ten years of operation, the UA2 experimental program stopped running at the end of 1990.

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. 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%.

The 1985 1987 upgrade of the detector was aimed at two aspects: full calorimeter coverage and better electron identification at lower transverse momenta. The first aspect was addressed by replacing the end caps with new calorimeters that covered the regions 6° 40° with respect to the beam direction, thereby hermetically sealing the detector. The end cap calorimeters consisted of lead/scintillator samplings for the electromagnetic part, and iron/scintillator for the hadronic part. This detector was built as a cylinder, closely surrounding the beam pipe. The detector had to fit into the available space of less than 1 cm. It was therefore necessary to miniaturize the components of the detector. This was achieved with two brand new technologies: the silicon sensor and the Application Specific Integrated Circuit (ASIC). Existing electronics were too bulky, and therefore a novel ASIC had to be developed. This was the first silicon tracker adapted to a collider experiment, a technology prior to the present silicon detectors. Observations of hadronic jets confirmed that the theory of quantum chromodynamics could describe the gross features of the strong parton interaction. The next step was to track down the Z boson. However, the theory said that the Z boson would be ten times rarer than the W boson. The experiments therefore needed to collect several times the data collected in the 1982 run that showed the existence of the W boson. With improved techniques and methods, the luminosity was increased substantially. These efforts were successful, and on 1 June 1983, the formal announcement of the discovery of the Z boson was made at CERN.

Throughout the runs with the upgraded detector, the UA2 collaboration was in competition with experiments at Fermilab in the US in the search for the top quark. Physicists had anticipated its existence since 1977, when its partner — the bottom quark — was discovered. It was felt that the discovery of the top quark was imminent. During the 1987 1990 run UA2 collected 2065 decays, and 251 Z decays to electron pairs, from which the ratio of the mass of the W boson and the mass of the Z boson could be measured with a precision of 0.5%.