UA2 experiment

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.

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.

[4] 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).

[6] 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 initial UA2 collaboration consisted of about 60 physicists from Bern, CERN, Copenhagen, Orsay, Pavia and Saclay.

[3] The UA2 sub-detectors were also upgraded, making the detector hermetic, which increased its ability to measure missing transverse energy.

Groups from Cambridge, Heidelberg, Milano, Perugia and Pisa joined the collaboration, which grew to about 100 physicists.

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 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.

These showers passed through layers of plastic scintillators, generating light which was read with photomultiplier by the data collection electronics.

[10] The 1985-1987 upgrade of the detector was aimed at two aspects: full calorimeter coverage and better electron identification at lower transverse momenta.

[11] 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.

This was achieved with two brand new technologies: the silicon sensor and the Application Specific Integrated Circuit (ASIC).

[13] Observations of hadronic jets confirmed that the theory of quantum chromodynamics could describe the gross features of the strong parton interaction.

[16] 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.

The UA2 detector shown in open position at the CERN Proton-Antiproton Collider in 1982
Civil engineering for the underground experimental hall at LSS4
Detector for the UA2 experiment. The picture shows the detector after the 1985-1987 upgrade, when new end-cap calorimeters were added to improve the search for the top quark and new physics.
Press conference on 25 January 1983 when the announcement was made of the discovery of the W boson at CERN . From right to left: Carlo Rubbia , spokesperson of the UA1 experiment ; Simon van der Meer , responsible for developing the stochastic cooling technique; Herwig Schopper , Director-General of CERN; Erwin Gabathuler , Research Director at CERN, and Pierre Darriulat , spokesperson of the UA2 experiment.