frequent, have a larger background. By the end 1982, the SpS had reached high enough luminosity to permit the observation of and decays. On 22 January 1983, the UA2 collaboration announced 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, CERN made a public announcement that the W boson was found. 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. Search for the top quark 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%. By 1991 a precise measurement for the mass of the Z boson from LEP had become available. Using the ratio of the W mass to Z mass, a first precise measurement of the W mass could be made. These mass values could be used to predict the top quark from its virtual effect on the W mass. The result of this study gave a top quark mass value in the range of 110 GeV to 220 GeV, beyond the reach for direct detection by UA2 at the SpS. The top quark was ultimately discovered in 1995 by physicists at Fermilab with a mass near 175 GeV. See also List of Super Proton Synchrotron experiments References External links Archive of the UA2 Collaboration Darriulat, Pierre (2004). "The W and Z particles: a personal recollection", CERN Courier Di Lella, Luigi; Rubbia,