Measurement of the top quark pair production cross section in proton-antiproton collisions at a center of mass energy of 1.96 TeV, hadronic top decays with the D0 detector

Abstract

Of the six quarks in the standard model the top quark is by far the heaviest: 35 times more massive than its partner the bottom quark and more than 130 times heavier than the average of the other five quarks. Its correspondingly small decay width means it tends to decay before forming a bound state. Of all quarks, therefore, the top is the least affected by quark confinement, behaving almost as a free quark. Its large mass also makes the top quark a key player in the realm of the postulated Higgs boson, whose coupling strengths to particles are proportional to their masses. Precision measurements of particle masses for e.g. the top quark and the W boson can hereby provide indirect constraints on the Higgs boson mass. Since in the standard model top quarks couple almost exclusively to bottom quarks (t → Wb), top quark decays provide a window on the standard model through the direct measurement of the Cabibbo-Kobayashi-Maskawa quark mixing matrix element Vtb. In the same way any lack of top quark decays into W bosons could imply the existence of decay channels beyond the standard model, for example charged Higgs bosons as expected in two-doublet Higgs models: t → H+b. Within the standard model top quark decays can be classified by the (lepton or quark) W boson decay products. Depending on the decay of each of the W bosons, t$\bar{t}$ pair decays can involve either no leptons at all, or one or two isolated leptons from direct W → e$\bar{v}${sub e} and W → μ$\bar{v}$μ decays. Cascade decays like b → Wc → e$\bar{v}$ec can lead to additional non-isolated leptons. The fully hadronic decay channel, in which both Ws decay into a quark-antiquark pair, has the largest branching fraction of all t$\bar{t}$ decay channels and is the only kinematically complete (i.e. neutrino-less) channel. It lacks, however, the clear isolated lepton signature and is therefore hard to distinguish from the multi-jet QCD background. It is important to measure the cross section (or branching fraction) in each channel independently to fully verify the standard model. Top quark pair production proceeds through the strong interaction, placing the scene for top quark physics at hadron colliders. This adds an additional challenge: the huge background from multi-jet QCD processes. At the Tevatron, for example, t$\bar{t}$ production is completely hidden in light q$\bar{q}$ pair production. The light (i.e. not bottom or top) quark pair production cross section is six orders of magnitude larger than that for t$\bar{t}$ production. Even including the full signature of hadronic t$\bar{t}$ decays, two b-jets and four additional jets, the QCD cross section for processes with similar signature is more than five times larger than for t$\bar{t}$ production. The presence of isolated leptons in the (semi)leptonic t$\bar{t}$ decay channels provides a clear characteristic to distinguish the t$\bar{t}$ signal from QCD background but introduces a multitude of W- and Z-related backgrounds.

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