No description available.

The LHC and the ATLAS detector offer an unprecedented opportunity to test theories beyond the Standard Model of particle physics. Some of these theories predict heavy particles that would decay predominantly into a top and an antitop quark. This thesis presents a technique to experimentally measure the invariant mass (Mtt) of top-antitop pairs, event-by-event, allowing for a complete reconstruction of the Mtt spectrum. Two different models of heavy narrow resonances were considered: a particle X with a negligible mass width, and a Z' particle. Under these models, no resonances in the Mtt spectrum were found in 35.3 pb-1 of 7 TeV centre of mass proton-proton collision data. An upper limit on the production cross section times the branching ratio of the heavy particle decaying into a top-antitop (Sup) as a function of its mass (Mx or Mz) was established at a 95% confidence level. Including systematic errors, the observed (expected) Sup at 95% varies from 3.2 pb (27.9 +11.3 -19.9 pb) to 77.7 pb (9.8 +10.7 -5.6 pb) for Mx ranging from 760 GeV to 1000 GeV, and from 2.9 pb (55.9 +56.9 -47.0 pb) to 43.4 pb (7.8 +8.9 -4.7 pb) for Mz ranging from 700 GeV to 1000 GeV. / Graduate

Eine Messung der W-Boson Polarisation in Topquark-Zerfaellen wird vorgestellt. Es wurden Topquarks untersucht, welche in Proton-Proton Kollisionen bei einer Schwerpunktsenergie von sqrt{s} = 7 TeV am LHC erzeugt wurden. Der Datensatz basiert auf einer integrierten Luminositaet von 4.7 fb-1 und wurde mit dem ATLAS-Detektor gesammelt. Die Messung wurde im Lepton+Jets Kanal durchgefuehrt, welcher durch ein isoliertes Elektron oder Myon, fehlende transversale Energie und mindestens vier Jets charakterisiert wird. Einer der Jets muss als b-Jet identifiziert sein. Zum Studium der W-Boson Polarisation wurde die Winkelverteilung des geladenen Leptons im Ruhesystem des W-Bosons verwendet. Das Ereignis wurde vollstaendig mit Hilfe eines kinematischen Likelihood-Fits rekonstruiert. Die Anteile der linkshaendigen, rechtshaendigen und longitudinal polarisierten W-Bosonen wurden durch einen gebinnten Likelihood-Fit an die Datenverteilung bestimmt. Ein kombinierter Likelihood-Fit wird durchgefuehrt, um die W-Helizitaetsanteile fuer den Lepton+Jets Kanal zu bestimmen.

The LHC and the ATLAS detector offer an unprecedented opportunity to test theories beyond the Standard Model of particle physics. Some of these theories predict heavy particles that would decay predominantly into a top and an antitop quark. This thesis presents a technique to experimentally measure the invariant mass (Mtt) of top-antitop pairs, event-by-event, allowing for a complete reconstruction of the Mtt spectrum. Two different models of heavy narrow resonances were considered: a particle X with a negligible mass width, and a Z' particle. Under these models, no resonances in the Mtt spectrum were found in 35.3 pb-1 of 7 TeV centre of mass proton-proton collision data. An upper limit on the production cross section times the branching ratio of the heavy particle decaying into a top-antitop (Sup) as a function of its mass (Mx or Mz) was established at a 95% confidence level. Including systematic errors, the observed (expected) Sup at 95% varies from 3.2 pb (27.9 +11.3 -19.9 pb) to 77.7 pb (9.8 +10.7 -5.6 pb) for Mx ranging from 760 GeV to 1000 GeV, and from 2.9 pb (55.9 +56.9 -47.0 pb) to 43.4 pb (7.8 +8.9 -4.7 pb) for Mz ranging from 700 GeV to 1000 GeV. / Graduate

No description available.

No description available.

Le but premier de la physique des hautes énergies est d'améliorer notre connaissance de la structure fondamentale de la matière, et notamment des particules qui constituent le monde. L'une d'entre elles est le quark top, qui fut découvert en 1995 par les collaborations CDF et D0 auprès du collisionneur protons-antiprotons Tevatron. Un des buts principaux du Tevatron a été depuis l'étude fine des propriétés du quark top, et en particulier de la section efficace de production de paires top-antitop. Différentes analyses ont été menées dans les canaux leptons, dileptons, et tout hadronique afin de déterminer le plus précisément possible les valeurs de ces paramètres, et ainsi de tester la validité du Modèle Standard. Le but principal de cette thèse est de vérifier une des prédictions théoriques du Modèle Standard de la physique des particules, à savoir la section efficace de production top-antitop, auprès du collisionneur Tevatron. Le canal étudié est constitué d'un muon, d'un lepton tau, de leurs neutrinos associés, de de deux jets de quark b (l'un d'entre eux provenant d'un quark b, l'autre d'un anti-quark b). La reconstruction et l'identification des jets , des muons, des leptons taus (en particulier par l'utilisation d'un réseau de neurones dédié), et l'étiquetage des jets de b, représentent les éléments fondamentaux de cette étude. Au final, un bon accord entre les données et la simulation Monte Carlo est obtenu à la dernière étape de cette analyse. En outre, la mesure finale de la prédiction théorique au niveau NLL + NLO du calcul des pertubations sont compatibles. / The purpose of high energy physics is to improve our knowledge about the fundamental structure of matter, in particular about particles that constitute the world. One of these is the top quark, that was discovered in 1995 by the CFD and D0 collaborations at the Tevatron protons-antiprotons collider. One of the primary aim of the Tevatron has been then the fine study of the top quark propertiers, in particular the top-antitop production cross section. SDifferent analysis have been performed in the leptons, dileptons, and all-hadronic channels to determine accurately the values of these parmatersn and thus to test the validity of the Standard Model.The main goal of this thesis is to verify one of the theoretical predictions of the Standard Model of particle physics, the top-antitop production cross section, at the Tevatron collider. The channel studied is constituted by one muon, one tau lepton, their associated neutrinos, and two b-jets (one coming from the quark, the other one coming from a b anti-quark). The reconstruction and identification of jets, of muons, of taus (in particular with the use of a taus neural network), and the b-tagging of jets, represent the fundamental elements of this study. Endly, a good agreement between data and Monte Carlo simulation is obtained at the final stage of analysis. Moreover, the final measurement and the theoretical prediction at the NLL+NLO perturbations level are found to be compatible.

No description available.

No description available.

No description available.