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Recherche de la production électrofaible du quark top dans le canal électron+jets dans l'expérience D0 auprès du Tevatron

Abstract : The top quark, whose mass approches the electroweak symmetry breaking scale, is by far the heaviest known elementary particle. New physics is therefore expected to have its most important effect in the top sector. The Tevatron is, currently, the only collider able to produce the top quark. Among all possible production processes in the standard model, the top-antitop pair production via strong interaction, first observed in 1995, is the one with the largest cross section. The production via electroweak interaction (known as single top production), more difficult to extract from the background because of a lower cross section and of a lower signal to background ratio, has never been observed. In this thesis, we have searched for these processes by studying proton-antiproton collisions at sqrt(s)=1.96 TeV produced by the Tevatron and detected with the D0 detector. The experimental study of the top quark is very sensitive to the quality of the data taken by the calorimeter. This detector showed, at the beginning of the Run II, rather important noise problems. Having identified the origin of the noise, new treatments at the offline level were implemented and their effects studied. It has been shown that these treatments reduce very significantly the effect of the noise in the reconstruction of physical quantities without notable degradation of the signal. Within the standard model, the top quark decays into Wb with a branching ratio close to 100%. Leptonic decays of the W into electron + neutrino have been used to identify the W from the top decay. The main backgrounds to the single top signal (W+jets and QCD) are made essentially of light quark jets in the final state. Two b-tagging algorithms have therefore been applied in order to improve the signal to background ratio. No evidence for electroweak top quark production has been found. Upper limits at the 95% confidence level on the observed (expected) cross sections have be computed. They are found to be 14.3 (11.3) pb for the s-channel, 27.7 (21.5) pb for the t-channel and 28 (19.8) pb for the sum of the two channels.
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Contributor : Emmanuel Busato <>
Submitted on : Friday, July 1, 2005 - 3:09:49 PM
Last modification on : Friday, April 10, 2020 - 5:29:27 PM
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Emmanuel Busato. Recherche de la production électrofaible du quark top dans le canal électron+jets dans l'expérience D0 auprès du Tevatron. Physique des Hautes Energies - Expérience [hep-ex]. Université Paris-Diderot - Paris VII, 2005. Français. ⟨tel-00009641⟩

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