Le travail de cette thèse a été mené au sein de la collaboration CMS (Compact Muon Solenoid), une des quatre experiences en cours d'installation au LHC (Large Hadron Collider) du CERN. La thèse se focalise sur le calorimètre électromagnétique de CMS: les performances du detecteur sont étudiées en détail en utilisant soit des données de tests réalisés sur faisceau d'électrons soit une simulation complète du détecteur. Une étude du bruit de fond électronique dans la chaîne de lecture a conduit à l'élaboration de deux méthodes de reconstruction de l'amplitude des signaux, l'une en cas de saturation de la chaîne et l'autre en cas de bruit de fond cohérent. La thèse presente aussi une étude des méthodes de reconstruction des électrons dans CMS, qui a conduit à la première définition dans CMS de la qualité de mesure d'un électron, à une procédure pour établir l'échelle d'énergie des électrons et à la combinaison des informations du calorimètre et du trajectomètre pour obtenir une estimation optimale de l'impulsion de l'électron au vertex. Ces resultats sont appliqués à l'analyse du potentiel de découverte de l'Higgs dans le canal de désintegration H->ZZ(*)->4e pour lequel une te! chnique standard de selections séquentielles ainsi qu'une technique optimale utilisant des reseaux de neurones sont proposées.

Le LHC est un accélérateur de hadrons conçu essentiellement pour la découverte du boson de Higgs supposé être à l'origine de la masse des particules élémentaires. Ce boson est cependant le maillon manquant au Modèle Standard. Au LHC, les protons seront produits avec une énergie de 7 TeV chacun et la luminosité totale atteindra 10^34 cm-2 s-1. CMS est l'une des quatre grandes expériences actuellement en construction autour du LHC. Sa partie centrale sera constituée de senseurs au silicium à micro-pistes. Ces détecteurs au silicium sont, à présent, les plus précis pour la reconstruction des trajectoires des particules chargées. Néanmoins, les conditions de fonctionnement du LHC créent un environnement très hostile qui pourrait affecter à long terme les performances du silicium ainsi que l'électronique qui lui est associée. De ce fait, il devient judicieux de contrôler les effets des radiations et de s'assurer du bon fonctionnement de ces détecteurs pour toute la durée de leur opération. Cette thèse s'inscrit dans le cadre des tests de résistance aux radiations des senseurs à micro-pistes de CMS. Une présentation générale de CMS, des senseurs au silicium à micro-pistes ainsi que des effets des dégâts radiatifs sur ces derniers sera donnée en premier lieu. Cette présentation sera suivie d'un descriptif des dispositifs d'irradiation et de caractérisation électrique développés spécialement pour tester ces senseurs. Enfin, les résultats obtenus seront présentés et discutés.

We present a measurement of the top anti-top quark (ttbar)production cross section in the dilepton final states from proton-proton collisions at a center of mass energy at 7 TeV at the LHC. A b-tagging algorithm based on tracks displaced from the event interaction vertex is applied to identify bottom quark jets from top quark decay and reject background events. Given the relatively pure sample of bottom quark jets in ttbar dilepton final states, a new technique to measure in-situ the b-tagging efficiency is introduced that uses the distribution of the number of observed b-tagged jets. We present results with data collected at the ATLAS detector in 2010 with an integrated luminosity of 35 pb-1. The measured ttbar cross section is 176 +22/-21 (stat.) ± 20 (syst.) ± 6 (lum.) pb in the dilepton channel. We will also discuss the future prospects of this measurement.

We present a measurement of the top anti-top quark (ttbar)production cross section in the dilepton final states from proton-proton collisions at a center of mass energy at 7 TeV at the LHC. A b-tagging algorithm based on tracks displaced from the event interaction vertex is applied to identify bottom quark jets from top quark decay and reject background events. Given the relatively pure sample of bottom quark jets in ttbar dilepton final states, a new technique to measure in-situ the b-tagging efficiency is introduced that uses the distribution of the number of observed b-tagged jets. We present results with data collected at the ATLAS detector in 2010 with an integrated luminosity of 35 pb-1. The measured ttbar cross section is 176 +22/-21 (stat.) ± 20 (syst.) ± 6 (lum.) pb in the dilepton channel. We will also discuss the future prospects of this measurement.

In the ATLAS experiment software plays a central role in data taking, reconstruction, and analysis. This thesis presents a set of analysis algorithms written for the ATLAS software framework Athena, towards H+ searches in the τ+jets channel. Data recorded by the ATLAS experiment for 900 GeV proton-proton collisions from 2009 and 7 TeV proton-proton collisions from 2010 are analyzed with these algorithms and compared to Monte Carlo. A search for top quarks is performed on 36 pb-1 of data, and the results are found to agree well with Monte Carlo.

Non-evaporable getters (NEG) are widely used in ultra high vacuum (UHV) systems for particle accelerators to assure distributed pumping speed. By heating the NEG to an activation temperature, the oxide layer on the surface dissolves into the material, leaving a clean (activated) surface. The activated NEG surface is capable of chemisorbing most of the residual gases present in a UHV system and will act as a vacuum pump. NEG can be sputter deposited on the inner wall of vacuum chambers, turning the whole wall from a source of gas into a pump. At the largest particle accelerator in the world, the Large Hadron Collider, more than 6 km of beam pipe has been NEG coated. In this work, a DC magnetron sputtering system dedicated for coating cylindrical vacuum chambers with NEG has been assembled, installed and commissioned. The system has been used to do NEG depositions on inner walls of vacuum chambers. The vacuum performance of the coating has been measured in terms of pumping speed, electron stimulated desorption and activation temperature. In addition, the thin film composition and morphology has been investigated by scanning electron microscopy (SEM). The work has resulted in an operational DC magnetron sputtering system, which can be used for further studies of NEG materials and compositions.

<p>A simultaneous measurement of three Standard Model cross-sections using 4.7 inverse femtobarns of proton-proton collision data at a center-of-mass energy of 7 TeV is presented. Collision data were collected using the ATLAS detector at the Large Hadron Collider. The signal production cross-sections studied are for top quark pair production, charged weak boson pair production, and Drell-Yan production of tau lepton pairs with invariant mass greater than 40 GeV. A data sample is defined from events with isolated high-energy electron-muon pairs arranged in a phase space defined by missing transverse momentum and jet multiplicity. A binned maximum likelihood fit is employed to determine signal yields in this phase space. Signal event yields are in turn used to measure full cross-section values and cross-section values within a fiducial region of the detector, and unlike conventional measurements the signal measurements are performed simultaneously. This is the first such simultaneous measurement of these cross-sections using the ATLAS detector. Measured cross-sections are found in good agreement with the most precise published theoretical predictions.</p> / Dissertation

The Large Hadron Collider (LHC) at the international particle physics laboratory CERN in Switzerland is currently the most powerful particle accelerator on earth. This thesis presents analyses of proton-proton collisions at the energy √s = 7 TeV, recorded by ATLAS, one of the detectors at the LHC. The goal of the LHC and its detectors is to find new phenomena not described by the Standard Model (SM) of particle physics.The top quark is the heaviest known elementary particle and it is produced in very large numbers at the LHC. Measuring the production cross-section of top pairs (ttbar) is important for many reasons: for validating the strong production mechanism of the SM, for commissioning and calibration of the detector and analysis software and because several scenarios for physics beyond the SM predict changes to the ttbar production cross-section.Five different measurements of the ttbar cross-section will be presented in this thesis. The first three are measurements of the total cross-section, the fourth is a simultaneous measurement of the ttbar, Zττ and WW cross-sections and the fifth is a measurement of the relative differential ttbar cross-section. The most accurate measurement of the total cross-section is 176 pb with a total uncertainty of 9%, and the relative differential cross-section for ttbar-masses above ~ 1 TeV is 0.007 1/TeV with an uncertainty of 43%. Both values agree with the SM predictions.Measurements or searches in particle physics often have to be conducted in the presence of uninteresting background processes.  Reducing and providing estimates of these backgrounds is one of the main analysis tasks. Many backgrounds can be simulated with sufficiently good accuracy. However, the background due to mis-identified leptons cannot be accurately simulated. This thesis presents and evaluates a method for estimating this background from data, and this is then used in the total ttbar cross-section measurements. / LHC (Large Hadron Collider) vid det internationella partikelfysiklaboratoriet CERN i Schweiz är för närvarande världens mest kraftfulla partikleaccelerator. I den här avhandlingen presenteras anlyser av proton-protonkollisioner vid $\sqrt{s}=7\TeV{}$, registrerade med ATLAS som är en av detektorerna vid LHC. Målet med LHC och dess detektorer är att upptäcka nya fenomen som inte kan beskrivas av partikelfysikens nuvarande standardmodell (SM). Toppkvarken är den tyngsta elementarpartikeln man känner till och den produceras i stort antal vid LHC. Att mäta produktionstvärsnittet för par av topkvarkar (\ttbar{}) är viktigt av flera skäl: för att validera den starka produktionsmekanismen i SM, och för att testa och kalibrera detektorn och analysverktygen. Dessutom förutsäger flera teoretiska utvidgningar av SM ändringar av tvärsnittet för \ttbar{}. Fem olika mätningar av tvärsnittet för \ttbar{} presenteras i den här avhandlingen. De första tre är mätningar av det totala tvärsnittet, den fjärde är en simultan mätning av tvärsnitten för produktion av \ttbar{}, $WW$ och \Ztt{} och den femte är en mätning av det \emph{relativa differentiella} tvärsnittet. Det mest noggranna mätningen av det totala tvärsnittet är $176 $ pb med $9 \%$ total osäkerhet, och det relativa differentiella tvärsnittet för \ttbar{}-massor över $\sim 1 \TeV{}$ är $0.007 \TeV^{-1}$ med $43\%$ osäkerhet. Båda mätresultaten stämmer med respektive förutsägelse från standardmodellen. Mätningar eller sökningar inom partikelfysiken måste ofta utföras i närvaro av ointressanta bakgrundsprocesser. Att reducera och skatta dessa bakgrunder är en av huvuduppgifterna inom en dataanalys. Många bakgrunder kan simuleras med tillräcklig precision. Men bakgrunden av felidentifierade leptoner kan däremot inte simuleras tillräckligt noggrant. Den här avhandlingen presenterar och utvärderar en metod för att skatta bakgrunden av felidentifierade leptoner från data, och resultatet används i mätningarna av det totala tvärsnittet för \ttbar{}. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 5: Manuscript.</p>

In this dissertation results are presented from a search for the pair production of heavy colored particles (gluinos, squarks) in R-parity conserving supersymmetric models, in which the lightest supersymmetric particle is a stable and neutral object. The search was performed for events with at least two tau leptons, two highly energetic jets and large missing transverse momentum in the final state on a data sample of proton-proton collisions at sqrt(s) = 7 TeV. The data sample was collected by the Compact Muon Solenoid detector at the Large Hadron Collider in 2011, and it corresponds to an integrated luminosity of 5fb^−1. The tau isolation variable was optimized for this search. The number of events corresponding to standard model processes in the final selection was estimated to be 7.49 ± 0.74 using background estimation techniques based on data. Nine observed events are found to be in agreement with the standard model prediction, and exclusion limits on gluino mass are obtained in the context of supersymmetric models at the 95% confidence level.

Techn. Hochsch., Diss., 2001--Aachen.