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%. Upgrades of the detector 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. The performance and granularity of the new calorimeters were set to match the central calorimeter, which was of importance for the triggering system. The electron identification was improved by the use of a completely new central tracking detector assembly, partly consisting of a pioneering silicone-pad detector. In 1989, the collaboration pushed this concept even further by developing a Silicon Pad Detector (SPD) with finer pad segmentation to be placed directly around the collision region beam pipe. 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. Results Hadronic jets at high transverse momentum The very first result of the UA2 collaboration, published on 2 December 1982, was the first unambiguous observation of hadronic jet production at high transverse momentum from hadronic collisions. Observations of hadronic jets confirmed that the theory of quantum chromodynamics could describe the gross features of the strong parton interaction. Discovery of the W and Z bosons The UA2 and UA1 collaboration chose to search for the W boson by identifying its leptonic decay, because the hadronic decays, although more