On Observing Top Quark Production at the Tevatron

A technique for separating top quark production from Standard Model background events is introduced. It is applicable to the channel in which one top quark decays semi-leptonically and its anti-quark decays hadronically into three jets, or vice versa. The method is shown to discriminate dramatically between Monte Carlo generated events with and without simulated top quarks of mass around 120 GeV and above. The simulations were performed with CDF detector characteristics incorporated, showing that the method is applicable to existing data. With the discoveries of the tau lepton and then of the bottom quark, it became clear that the completion of a third generation in the Standard Model required a sixth quark flavor, now named ”top”. The successes of this model are now so numerous and so detailed that few doubt the existence of the top quark[1], yet its discovery has remained elusive. The only firm knowledge that we have about its mass is that it exceeds 89 GeV[2], a bound deduced from the empirical di-lepton inclusive total cross-section corresponding to topantitop production followed by decays which include a lepton. Some theoretical arguments suggest that its mass may not be larger than about 200 GeV, and probably less. Unless its mass is much greater than this, top -antitop pairs should be produced by the Tevatron collider, but formidible backgrounds have made its detection quite difficult. Owing to its uncommonly large mass, top decay is expected to have some unique features[3,4]. The most important of these are that decay through the (V-A) weak interaction, to a b-quark and a real W boson will dominate, and that,if the top quark mass mt is above about 120 GeV, this weak decay will occur more rapidly than hadronization[3]. Recently, two of us [3] developed a scheme for analysing top-antitop pair production in protonantiproton annihilation through their leptonic decay modes. The two-step decay t→bW, W→ lν, led us to construct a paraboloid surface of possible top three-momenta, based on the momenta of the b-jet and the lepton. Each possible top energy corresponds to a circular cross-section of it; each possible top mass to an elliptic section of it. A tt̄ pair is produced by the interaction of a parton of the proton with an antiparton of the antiproton. It is known empirically that the partons have quite limited transverse momentum. Hence the simplest parton model requires that the net transverse momentum of the produced pair be zero; however, to leading order in QCD, some transverse momentum can arise from gluon bremsstrahlung or the emission of hard gluons. The result[3] is that a real tt̄ pair corresponds to points on the transverse plane projection of their two ellipses, such that there is near cancellation of their transverse momenta. This criterion has been used for the analysis of the CDF tt̄ candidate event [5], with the conclusion that the mass mt must be about 131 GeV[6], if this identification of the event is correct. Although this event was very striking, and well-fitted in this way, this analysis was not enough to prove that this event was an example of tt̄ pair production. The W also decays hadronically, through the two quark channel, more probably than leptonically (by a factor of about 6 to 1 for the number of q−q̄ combinations versus any single lepton channel). Hence a larger signal for top pair production is the event configuration