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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.

La recherche du Boson de Higgs du modèle standard produit en association avec un boson Z dans le canal ZH → e+e- bbˉ a été présentée. Cette étude a été réalisée avec 4.2 fb-1 de données collectées avec le détecteur DØ auprès du Tevatron. Aucun excès de donnée n'a été observé. Une limite sur la section efficace de production du processus ZH → e+e- bbˉ a été établie à 95% CL. / A search for a standard model Higgs boson in the ZH → e+e- bbˉ channel is presented using 4.2 fb-1 of data the DØ detector at the Tevatron. No excess of data have been found. A 95% CL limit on the ZH → e+e- bbˉ cross section production have been set.

Dans le Modèle Standard de la Physique des Particules, le boson de Higgs permet de générer la masse des particules élémentaires. Les contraintes théoriques et expérimentales actuels imposent au boson de Higgs d'avoir une masse comprise entre 114.4 et 158 GeV à 95% de niveau de confiance. Ces résultats confortent la recherche du boson de Higgs à basse masse dans la fenêtre encore ouverte.Le détecteur Dø est situé près de Chicago, au Tevatron, collisionneur protons-antiprotons avec une énergie dans le centre de masse de 1.96 TeV. le sujet de cette thèse est la recherche du boson de Higgs produit en association avec le boson Z. C'est un canal sensible au boson de Higgs de basse masse (<135 GeV) qui a un rapport avec le branchement H -> bb de l'ordre de 80% dans cette région en masse. Le canal d'étude ZH → nunubb a un état final composé de 2 jets de saveurs lourdes et de l'énergie transverse manquante emportée par les neutrinos.L'identification des jets de saveur lourde est réalisée à l'aide d'un nouvel algorithme que l'on a développé (SLTNN) : la méthode est basée sur la désintégration semi-leptonique des quarks b.L'analyse de recherche du boson de Higgs a été menée avec 3 fb-1 de données. L'utilisation de SLTNN a permis d'améliorer de 10% l'efficacité d'identification de boson de Higgs. En revanche, la sensibilité globale de l'analyse, une fois les bruits de fonds et erreurs systématiques prises en compte, est très peu améliorée. / In the Standard Model of particle physics, the Higgs boson generates elementary particles mass. Current theoretical and experimental constraints leads of a Higgs boson mass between 114.4 and 158 GeV with 95% confidence level. Moreover, Tevatron has recently excluded the mass window between 100 et 109 GeV with 95% confidence level. These results gives a clear indication search Higgs boson at low mass. Dø detector is located close to Chicago, at Tevatron, proton-antiproton collider with an energy in the center of mass 1.96 TeV. the topic of this thesis is the search of Higgs boson associated to a Z boson. It is a sensitive channel to low mass Higgs boson (<135 Gev) which has a branching ratio close to 80% in this mass range. The decay channel ZH → nunubb has in the final state 2 heavy-flavor jets and missing transverse energy due to neutrinos.The heavy-flavor jets identification is done thanks a new algorithm we have developped (SLTNN) : the method is based on semi-leptonic decay of b quarks.the Higgs boson search analysis was done with 3 fb-1 of data. The use of SLTNN permits to increase by 10% of Higgs boson tagging efficiency. On the other hand, global analysis sensitivity improvement, after taking into account the backgrounds and systematic errors, is low

The CP violating phase β^J/ψφ_s is measured in decays of B⁰_s → J/ψφ. This measurement uses 5.2 fb^-1 of data collected in √s = 1.96 TeV p‾p collisions at the Fermilab Tevatron with the CDF Run-II detector. CP violation in the B⁰_s-‾B⁰_sbar system is predicted to be very small in the Standard Model. However, several theories beyond the Standard Model allow enhancements to this quantity by heavier, New Physics particles entering second order weak mixing box diagrams. Previous measurements have hinted at a deviation from the Standard Model expectation value for β^J/ψφ_s with a significance of approximately 2σ. The measurement described in this thesis uses the highest statistics sample available to date in the B⁰_s → J/ψφ decay channel, where J/ψ → μ⁺ μ^- and φ → K⁺K^-. Furthermore, it contains several improvements over previous analyses, such as enhanced signal selection, fully calibrated particle ID and flavour tagging, and the inclusion of an additional decay component in the likelihood function. The added decay component considers S-wave states of KK pairs in the B⁰_s → J/ψK⁺K^- channel. The results are presented as 2-dimensional frequentist confidence regions for β^J/ψφ<sub >s</sub> and ΔΓ (the width difference between the B⁰_s mass eigenstates), and as a confidence interval for β^J/ψφ_s of [0.02,0.52] U [1.08, 1.55] at the 68% confidence level. The measurement of the CP violating phase obtained in this thesis is complemented by the world's most precise measurement of the lifetime τ_s = 1.53 ± 0.025 (stat.) ± 0.012 (syst.) ps and decay width difference ΔΓ = 0.075 ± 0 .035 (stat.) ± 0.01 (syst.) ps⁻¹ of the B⁰_s meson, with the assumption of no CP violation.

<p>This thesis presents two measurements of the top quark using 230 pb-1 of data taken with the D0 detector at the Tevatron accelerator. The first measurement determines the top pair production cross section at sqrt(s)=1.96 TeV in proton-antiproton collisions. In the standard model of particle physics the top quark decays almost exclusively into a W boson and a b quark. Candidate events are selected by requiring that at least one jet in the event is tagged with the secondary vertex algorithm. The measured top pair production cross section is:</p><p>8.6 +1.6-1.5 (stat. + syst.) +- 0.6 (lumi.) pb.</p><p>The second measurement uses the observed and predicted number of events with 0, 1 and 2 b-tagged jets to estimate the ratio R:</p><p>R = B(t->Wb) / B(t->Wq)</p><p>where q stands for any down-type quark. The measured value is</p><p>R = 1.03 +0.19-0.17 (stat. + syst.)</p><p>in good agreement with the standard model prediction of R=1. The result can be used to obtain a lower limit for R:</p><p>R > 0.61 (95% C.L.).</p>

This thesis presents two measurements of the top quark using 230 pb-1 of data taken with the D0 detector at the Tevatron accelerator. The first measurement determines the top pair production cross section at sqrt(s)=1.96 TeV in proton-antiproton collisions. In the standard model of particle physics the top quark decays almost exclusively into a W boson and a b quark. Candidate events are selected by requiring that at least one jet in the event is tagged with the secondary vertex algorithm. The measured top pair production cross section is: 8.6 +1.6-1.5 (stat. + syst.) +- 0.6 (lumi.) pb. The second measurement uses the observed and predicted number of events with 0, 1 and 2 b-tagged jets to estimate the ratio R: R = B(t-&gt;Wb) / B(t-&gt;Wq) where q stands for any down-type quark. The measured value is R = 1.03 +0.19-0.17 (stat. + syst.) in good agreement with the standard model prediction of R=1. The result can be used to obtain a lower limit for R: R &gt; 0.61 (95% C.L.).

Missing transverse energy is an important aspect of physics analyses at hadron collider detectors. While other particles can be identified by the energy they deposit in the detector, the presence of neutrinos and other theorized particles must be inferred by an energy imbalance. At the DØ experiment missing energy algorithms exist not only to calculate the missing energy in an event, but to distinguish between possible sources: detector measurement effects or unobserved particles. DØ scientists rely on these algorithms to produce reliable physics results. This thesis presents updates made in the past year to missing energy certification, the unclustered energy resolution, and the missing energy significance calculation. It describes a new processor which calculates missing momentum from tracks as well as development work toward an unclustered energy calibration.

<p>The mass of the $W$ boson is one of the most important parameters in the </p><p>Standard Model. A precise measurement of the $W$ boson mass, together </p><p>with a precise measurement of the top quark mass, can constrain the </p><p>mass of the undiscovered Higgs boson within the Standard Model </p><p>framework or give a hint for physics beyond the Standard Model. </p><p>This dissertation describes a measurement of the $W$ boson mass </p><p> through its decay into a muon and a neutrino using </p><p> $\approx$ 2.2 fb$^{-1}$ of $\sqrt{s} = 1.96$ TeV $p\bar{p}$ data taken </p><p> with the CDF II detector at Fermilab. We measure the $W$ boson mass </p><p> to be ($80.374 \pm 0.015_{\rm stat.} \pm 0.016_{\rm syst.}$) </p><p> GeV/c$^2$. This result, when combined with the $W$ mass </p><p> measurement in the electron channel, leads to the single most </p><p> precise $m_W$ value and greatly constrains the possible mass </p><p> range of the undiscovered Higgs boson.</p> / Dissertation

University, Diss., 2004--München.