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The first measurement of the spin correlation strength in pairs of top quarks at √s=8 TeV is presented, using data collected using the ATLAS detector, corresponding to an integrated luminosity of 20.3 fb⁻¹. Dileptonically decaying tt^- events are selected with two high-pT leptons and at least two jets, one of which is required to be b-tagged. The azimuthal angle between the two charged leptons, Δφ, is used to measure the degree of spin correlation of top quark pairs. The extracted value of fSM, which is a measure of the spin-correlation strength, is fSM=1.20 ± 0.14. This is the most precise spin-correlation measurement to date. Using the Δφ distribution, a search for Supersymmetric top squarks is performed, with the assumption that the top squarks decay to a top quark and a neutralino. No excess of events is observed and top squarks with masses between the top quark mass and 191 GeV are excluded at 95% confidence level, extending previous limits.

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Doctor of Philosophy / Department of Physics / Timothy A. Bolton / This dissertation describes a search for the pair production of scalar top quarks, tilde[subscript] 1, using a luminosity of 995 pb[superscript-1] of data collected in p[bar over]p collisions with the D0 detector at the Fermilab Tevatron Collider at a center-of-mass energy sqrt[s] = 1.96 TeV. Both scalar top quarks are assumed to decay into a charm quark and a neutralino, [tilde chi superscript 0 subscript 1]where [(tilde chi superscript 0 subscript 1)]is the lightest supersymmetric particle. This leads to a final state with two acoplanar charm jets and missing transverse energy. The yield of such events in data is found to be consistent with the expectations from known standard model processes. Sets of tilde [subscript 1] and tilde chi [superscript 0 subscript 1] masses are excluded at the 95% confidence level that substantially extend the domain excluded by previous searches. With the theoretical uncertainty on the pair production cross section taken into account, the largest limit for m[subscript tilde subscript 1] is m[subscript tilde chi subscript 1> 150 GeV, for m [subscript tilde chi negative 0 subscript 1]= 65 GeV.

This thesis presents measurements of the the spin correlation strength in top anti-top quark pair production at the LHC using the ATLAS detector. The data used corresponds to 4.6 fb−1 of integrated luminosity taken during 2011 at the LHC at a center of mass energy of 7 TeV. The spin correlation is studied utilising different observables with different sensitivities to the production mechanism, in particular to gluon-gluon fusion in the like or unlike helicity state, quark anti- quark annihilation in the unlike helicity state, or a combination of the three. In addition cuts are made on the invariant mass of the ttbar system to enhance or suppress contributions from different initial state production mechanisms. The analysis presented is a precision test of both ttbar production and decay in the SM. These measurements are compared to the most current theoretical predictions. No deviation from the SM expectation was observed. In a subset of the data, corresponding to an integrated luminosity of 2.1 fb−1, the hypothesis of zero spin correlation is excluded at 5.1 standard deviations.

Cette thèse présente une étude de la production associée du boson de Higgs en association avec des quarks tops, suivie d’une désintégration du boson de Higgs en leptons tau, dans des collisions proton-proton (pp) à une énergie dans le centre de masse de sqrt(s) = 13 TeV enregistrées avec le détecteur CMS au Grand collisionneur de hadrons (LHC) du CERN.Ce travail s’inscrit dans le contexte du Run 2 du LHC, marqué par une augmentation de l’énergie dans le centre de masse de 8 à 13 TeV accompagnée d’une augmentation de la luminosité instantanée des collisions par rapport au Run 1. Pour faire face à ces nouvelles conditions de prise de données, CMS a entrepris une amélioration complète du système de déclenchement de niveau 1 accomplie avant 2016. Ce nouveau système et en particulier le nouvel algorithme électron et photon de niveau 1 ont contribué avec succès à un grand nombre de résultats CMS basés sur les données du Run 2. La mise en service de ce nouveau système ainsi que ces performances mesurées dans les premières données collectées par ce nouveau système de déclenchement sont présentées en détails.Une technique d’analyse novatrice basée sur la Méthode des Éléments de Matrice optimisée pour la recherche du processus ttH, H->tautau est également présentée, ainsi que plusieurs outils génériques pouvant âtre utilisés dans une large variété d’analyses H->tautau. Les résultats de l’analyse CMS utilisant cette méthode basés sur 35.9 fb-1 collectés en 2016 sont présentés. L’hypothèse "bruit de fond seul" est défavorisée mais n’est pas encore exclue par cette analyse à elle seule. / This thesis reports a study of the Higgs boson production in association with top quarks and decaying into τ leptons in proton-proton (pp) collisions at a center-of-mass energy of sqrt(s) = 13 TeV recorded with the CMS detector at the CERN Large Hadron Collider (LHC).This work has been carried out in the context of the Run 2 of the LHC, marked by an increase in the center-of-mass energy from 8 to 13 TeV together with an increase in the instantaneous luminosity of the collisions with respect to Run 1. To cope with this new data-taking conditions, CMS had initiated a full upgrade of the Level-1 trigger system achieved by 2016. This new system and in particular the new Level-1 electron and photon algorithm have successfully contributed to a large number of CMS results using Run 2 data. The commissioning of this new system and its performance measured with the first data collected with this new trigger are presented in details.A novel analysis technique based on the Matrix Element Method optimized for the search of the ttH, H->tautau process is also presented, including several generic tools which can be used in a large variety of H->tautau analyses. Results of the CMS analysis using this method based on 35.9 fb-1 collected in 2016 are presented. The background-only hypothesis is disfavored but not yet excluded by this analysis alone.

Diese Arbeit behandelt die Suche nach neuen Teilchen, die in Top-Quark-Paare zerfallen (t¯t). Die Analyse beruht auf Daten des ATLAS-Experiments von Proton- Proton-Kollisionen am LHC bei einer Schwerpunktsenergie von p s = 7 TeV und einer Gesamtluminosität von 2.05 fb−1. Hierzu wird der Lepton+Jets Endzustand im t¯t ! WbWb Zerfallskanal verwendet, worin ein W-Boson leptonisch und das andere hadronisch zerfällt. Das t¯t -Ereignis wird sowohl in aufgelösten als auch geboosteten Zerfallstopologien rekonstruiert. Zum ersten Mal werden die Korrelationen beider Kanäle in Form einer dritten Kategorie nutzbar gemacht, welche aus Ereignissen besteht, die in beiden Topologien selektiert wurden. Die Sensitivität der Analyse wird hierdurch erhöht. Obere Schranken bei 95% Vertrauensniveau auf den Wirkungsquerschnitt multipliziert mit der Zerfallsbreite für massive Zustände großer und kleiner Zerfallsbreite werden berechnet. Diese werden aus der Kombination der beiden Ansätze der t¯t -Rekonstruktion gewonnen. Für die Z0-Resonanz kleiner Breite reicht die beobachtete (erwartete) obere Grenze auf den Wirkungsquerschnitt von 4.85 (4.81) pb, für eine Masse von 0.6 TeV, bis 0.21 (0.13) pb, für eine Masse von 2 TeV. Eine schmale leptophobische Topcolor-Z0-Resonanzen mit einer Masse unterhalb von 1.3 TeV kann ausgeschlossen werden. Weiterhin konnten beobachtete (erwartete) obere Grenzen auch für eine breite Farboktett-Resonanz berechnet werden. Diese liegen zwischen 2.52 (2.59) pb und 0.37 (0.27) pb für Massen von 0.7 TeV bzw. 2 TeV. Breite Kaluza-Klein-Gluon-Resonanzen mit einer Masse unter 1.65 TeV können ausgeschlossen werden. / This thesis presents the search for new particles that decay into top quark pairs (t¯t). The analysis is performed with the ATLAS experiment at the LHC, using an integrated luminosity of 2.05 fb−1 of proton–proton collision data, collected at a center-of-mass energy of p s = 7 TeV. The lepton plus jets final state is used in the t¯t ! WbWb decay, where one W boson decays leptonically and the other hadronically. The t¯t system is reconstructed using both resolved and boosted topologies of the top-quark decay. For the first time, correlations between the two search channels have been employed by creating a third channel with the events selected by both analyses. The sensitivity to new physics phenomena is thereby improved. Upper limits are derived on the production cross-section times branching ratio for narrow and wide massive states, at the 95 % confidence level. These are extracted by combining the two approaches of the t¯t reconstruction. For a narrow Z0 resonance, the observed (expected) upper limits range from 4.85 (4.81) pb for a mass of 0.6 TeV, to 0.21 (0.13) pb for a mass of 2 TeV. A narrow leptophobic topcolor Z0 resonance with a mass below 1.3 TeV is excluded. Observed (expected) limits are also derived for a broad color-octet resonance. They vary between 2.52 (2.59) pb and 0.37 (0.27) pb for a mass of 0.7 TeV and 2 TeV, respectively. The wide Kaluza-Klein gluon with a mass below 1.65 TeV is excluded. Another aspect of this thesis are performance studies of the level-1 jet trigger. Trigger efficiencies have been measured, using data collected by the ATLAS detector in 2010 at p s = 7 TeV. The turn-on curves obtained for a variety of jet triggers, showed good agreement between data and simulation in the plateau region. The efficiency results were used at the first stage of analyses for multi-jet cross-section measurements.

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A measurement of the mass of the top quark is presented, using top-antitop pair (t-tbar) candidate events for the lepton+jets decay channel. The measurement makes use of Tevatron proton-antiproton collision data at centre-of-mass energy 1.96 TeV, collected at the CDF detector. The top quark mass is measured by employing an unbinned maximum likelihood method where the event probability density functions are calculated using signal (t-tbar) and background (W+jets) matrix elements, as well as a set of parameterised jet-to-parton mapping functions. The likelihood function is maximised with respect to the top quark mass, the fraction of signal events, and a correction to the jet energy scale (JES) of the calorimeter jets. The simultaneous measurement of the JES correction (ΔJES) provides an in situ jet energy calibration based on the known mass of the hadronically decaying W boson. Using 578 lepton+jets candidate events corresponding to 3.2 inverse femtobarns of integrated luminosity, the top quark mass is measured to be 172.4 ± 1.4(stat+ΔJES) ± 1.3(syst) GeV/c², one of the most precise single measurements to date.

Le Grand Collisionneur de Hadrons (Large Hadron Collider, LHC) du CERN a redémarré au printemps 2015 pour trois ans (Run 2) avec une énergie dans le centre de masse de 13 TeV.Une mesure précise de l’efficacité de reconstruction des électrons avec le détecteur ATLAS au LHC, est présente dans la première partie de cette thèse en utilisant les données récoltées au cours de l’année 2015. Cela a permis d’extraire les rapports d’efficacité de reconstruction des électrons, lesquels sont utilisés par toutes les analyses d’ATLAS impliquant des électrons. Les résultats montrent d'une part une grande efficacité de reconstruction des électrons avec le détecteur ATLAS, et une bonne compréhension de ses performances d’autre part.La seconde partie de ce manuscrit est dédiée à la recherche de la production associée du boson de Higgs avec une paire de quarks top (bar), qui pourrait permettre une première mesure directe du couplage de Yukawa entre le boson de Higgs et le quark top. Une déviation dans la mesure par rapport aux prédictions du Modèle Standard serait une preuve manifeste de Nouvelle Physique. La signature de l’état final avec deux leptons de même charge électrique électrons ou muons) est examinée en utilisant les premiers 10\% du total de données attendues pour le Run 2. Une méthode améliorant l'estimation des événements avec de faux (non-prompt) leptons a développé}e et est discutée en détails dans ce rapport. / The Large Hadron Collider (LHC) at CERN restarted in spring 2015 for three years (Run2) at an unexplored center-of-mass energy of 13 TeV. A precise measurement of electron reconstruction efficiency in ATLAS, one of the two general purpose experiments of the LHC, is presented in the first part of this thesis with $Zee$ data sample recorded in 2015. This allows to extract scale factors between data and simulation that are used by all ATLAS physics analyses involving electrons. The results show the high ability of the ATLAS detector to reconstruct electrons from one hand and the good understanding of its performance on the other hand. The second part of the thesis is dedicated to a search for the Higgs boson production in association with a top quark pair (ttH), which could allow a first direct measurement of the top quark Yukawa couplingand could reveal new physics. The signature with two same-charge light leptons (electron or muon) without a hadronically decaying tau lepton final state is examined using the first 10% of the total expected Run2 dataset. Events with fake (non-prompt) leptons represent the main reducible background of this signature. An improved method to estimate it has been developped and is discussed in details in this thesis. Driving the total error, fake leptons background is found to be 1.5 to 3.6 times higher than in simulation and represent between 32 and 48% of the total background. The best-fit value of the ratio of observed and Standard Model cross sections of ttH production process, combining with other multilepton channels, is 2.5±0.7(stat)+1.1-0.9(syst), and an upper limit on this ratio of 4.9 (2.3 expected) is found at 95% confidence level.