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Top Quark Discovered

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Top quark discovered Schematic of a decay pattern for the top quark-antiquark pair formed when a high energy proton and antiproton collide. Analys­ ing the emerging particles, physicists can track back to the top quark dynamics. ine months after a careful N announcement of tentative evidence for the long-awaited sixth 'top' quark, physicists from the CDF and DO experiments at Fermilab's Tevatron proton-antiproton collider declared on 2 March that they had finally discovered the top quark. Last year (June 1994, page 1), the CDF experiment at the Tevatron reported a dozen candidate top events. These, said CDF, had all the characteristics expected of top, but the difficulties of extracting the tiny signal from a trillion proton-antiproton collisions made them shy of claiming a discovery. For its part, the compan­ ion DO Tevatron experiment reported a few similar events but were even more guarded about their interpreta­ tion as top quarks. Just after these hesitant announce­ ments, performance at the Tevatron improved dramatically last summer. After the commissioning of a new linear accelerator and a magnet realignment, the machine reached a new world record proton-antiproton collision luminosity of 1.28 x 1031 per sq cm per s, ten times that originally planned. Data began to pour in at an unprecedented rate and the data frustrated by its heaviness - the top is but nevertheless was consistent with sample grew to six trillion collisions. some 40 times the mass of its 'beau­ no top production. When other Luminosity has subsequently climbed tiful' partner. Not only is the top quark scientific discoveries are claimed on to 1.7 x 1031. the heaviest by far, but it is the only the basis of much more flimsy evi­ The top quark is the final letter in quark which has been actively dence, the objectivity of these the alphabet of Standard Model hunted. After the quarry was Tevatron experiment analyses was particles. According to this picture, all glimpsed last year, the net has now widely admired. matter is composed of six strongly- been closed. Meanwhile a lot more Tevatron interacting subnuclear particles, the The painstaking analysis of the top proton-antiproton collision data have quarks, and six weakly interacting quark data was the highlight at major been scrutinized. As well as the particles, the leptons. Both sextets meetings last summer. At the Glas­ 1992-1993 run (Run la) which pro­ are neatly arranged as three pairs in gow conference (October, page 2), vided the initial samples, data from order of increasing mass. Hans Jensen of CDF cautiously said the subsequent 1994-1995 run (Run The fifth quark, the 'beauty' or 'b' 'This gives evidence for the top quark lb) have become available. quark, was also discovered at but does not firmly establish its After carefully trimming the raw 6 Fermilab, back in 1977. Since then existence.' For DO, Paul Grannis trillion collision sample to 40 million physicists have been eagerly waiting stated their sample had a small promising events, this information for the top to turn up, but have been excess over expected background, was painstakingly sifted, looking for CERN Courier, April/ May 1995 1 Top quark discovered Below, the CDF experiment (seen here in the Right, at Fermilab's Tevatron proton-antiproton 'exploded' maintenance position) at Fermilab's collider, the DO detector (seen here in its Tevatron has seen 43 examples of top quark assembly position) has, with its partner production in six trillion proton-antiproton Tevatron experiment CDF, finally produced collisions. hard evidence for the long awaited sixth 'top' (Photos Fermilab) quark. proton-antiproton collision point. DO found 17 top events, including 3 dilepton pairs, against an estimated background of about 4. Careful analysis of the decay kinematics of a suitable subset of top events reveals the mass of the decaying top quark. Finding the expected patterns of top decay is one thing, but the mass analysis is another. The decay patterns would be meaningless if they did not point to a narrow mass band. CDF reports a mass of 176 GeV, with an overall uncertainty of ± 13 GeV. This overlaps with the 174 GeV ±10% claimed last year, and is totally compatible with the accumulated Standard Model data (November signs of the daughter particles into teristic confined 'jets' of particles. 1994, page 5). DO gives a slightly which the top quark decays. The two However this top decay channel is higher top quark mass of 199 ± 30 experiments see these events easily polluted by background, and to GeV, but still bracketing the Standard distributed among the different types clean up the sample the experiments Model prediction. of decay expected after a top quark- look for signs of the b-quark or for In physics-speak, the statistical antiquark pair is formed in a proton- special event patterns expected from significance of the new top quark antiproton annihilation. top production. The b-quark can be measurement is expressed in likeli­ There are several such decay recognized either by its characteristi­ hood units of about 4.7 standard signatures. One occurs when both cally short decay time (picked up as deviations for each detector. This the top quark and antiquark decay a short track stub between the means that the overall possibility of (each via a W boson) to produce an original proton-antiproton collision some fluctuation in the background electron and an electron-neutrino or and the secondary decay) or by its count is less than about one part in a a muon and a muon-neutrino. The (semileptonic) decay products. million for either experiment. resultant electron and/or muon CDF found 43 top events, 6 with The experiments expect to amass (dilepton) pairs provide a clean dilepton pairs, compared to an twice the current data by December, source of top signals. expected background of 8 events when the Tevatron will be temporarily More probable but less clean is the from other processes which could shut down for routine maintenance process in which one top quark (or 'fake' a top quark signal. Recent CDF and to prepare for upgrades to the antiquark) decays as above while its analysis has benefited from an experiments to work with the still partner gives, via a W, another quark improved microvertex detector which higher collision luminosities expected pair. These quarks produce charac- picks up particle tracks close to the in the era of Fermilab's Main Injector. 2 CERN Courier, April//May 1995 .
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